fix: mark used storage slots in L2ScrollMessenger

[Thegaram]

Purpose or design rationale of this PR

Describe your change. Make sure to answer these three questions: What does this PR do? Why does it do it? How does it do it?

#850 removed some storage variables from L2ScrollMessenger. If we add some new storage variables in a future upgrade, they might use the same slots and might end up with unexpected initial values. This PR marks these two slots as used.

Layout of old version (4a30566):

> forge inspect L2ScrollMessenger storage-layout --pretty
| Name                    | Type                        | Slot | Offset | Bytes | Contract                                       |
|-------------------------|-----------------------------|------|--------|-------|------------------------------------------------|
| _initialized            | uint8                       | 0    | 0      | 1     | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| _initializing           | bool                        | 0    | 1      | 1     | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __gap                   | uint256[50]                 | 1    | 0      | 1600  | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| _owner                  | address                     | 51   | 0      | 20    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __gap                   | uint256[49]                 | 52   | 0      | 1568  | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| _paused                 | bool                        | 101  | 0      | 1     | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __gap                   | uint256[49]                 | 102  | 0      | 1568  | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| _status                 | uint256                     | 151  | 0      | 32    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __gap                   | uint256[49]                 | 152  | 0      | 1568  | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| xDomainMessageSender    | address                     | 201  | 0      | 20    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| counterpart             | address                     | 202  | 0      | 20    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| feeVault                | address                     | 203  | 0      | 20    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __gap                   | uint256[47]                 | 204  | 0      | 1504  | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| isL2MessageSent         | mapping(bytes32 => bool)    | 251  | 0      | 32    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| isL1MessageExecuted     | mapping(bytes32 => bool)    | 252  | 0      | 32    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| l1MessageFailedTimes    | mapping(bytes32 => uint256) | 253  | 0      | 32    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| maxFailedExecutionTimes | uint256                     | 254  | 0      | 32    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |

Layout of new version (c543d6f):

> forge inspect L2ScrollMessenger storage-layout --pretty
| Name                 | Type                     | Slot | Offset | Bytes | Contract                                       |
|----------------------|--------------------------|------|--------|-------|------------------------------------------------|
| _initialized         | uint8                    | 0    | 0      | 1     | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| _initializing        | bool                     | 0    | 1      | 1     | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __gap                | uint256[50]              | 1    | 0      | 1600  | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| _owner               | address                  | 51   | 0      | 20    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __gap                | uint256[49]              | 52   | 0      | 1568  | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| _paused              | bool                     | 101  | 0      | 1     | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __gap                | uint256[49]              | 102  | 0      | 1568  | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| _status              | uint256                  | 151  | 0      | 32    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __gap                | uint256[49]              | 152  | 0      | 1568  | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| xDomainMessageSender | address                  | 201  | 0      | 20    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| counterpart          | address                  | 202  | 0      | 20    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| feeVault             | address                  | 203  | 0      | 20    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| rateLimiter          | address                  | 204  | 0      | 20    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __gap                | uint256[46]              | 205  | 0      | 1472  | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| isL2MessageSent      | mapping(bytes32 => bool) | 251  | 0      | 32    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| isL1MessageExecuted  | mapping(bytes32 => bool) | 252  | 0      | 32    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |
| __used               | uint256[2]               | 253  | 0      | 64    | src/L2/L2ScrollMessenger.sol:L2ScrollMessenger |

PR title

Your PR title must follow conventional commits (as we are doing squash merge for each PR), so it must start with one of the following types:

• fix: A bug fix

Deployment tag versioning

Has tag in common/version.go been updated or have you added bump-version label to this PR?

• No, this PR doesn't involve a new deployment, git tag, docker image tag
• Yes

Breaking change label

Does this PR have the breaking-change label?

• No, this PR is not a breaking change
• Yes

[github-actions]

LCOV of commit c543d6f during Contracts #34

Summary coverage rate:
  lines......: 51.6% (1013 of 1963 lines)
  functions..: 65.6% (227 of 346 functions)
  branches...: no data found

Files changed coverage rate: n/a