Update the default mempool max tx bytes past 8MB

[rootulp]

          Can we have an issue to bump this value past 8MB

Originally posted by @cmwaters in #3961 (comment)

[rootulp]

For context celestia-app overrides the defaults:

	cfg.Mempool.MaxTxBytes = 7_897_088
	cfg.Mempool.MaxTxsBytes = 39_485_440

7_897_088 bytes is 7.5 MiB. Given Arabica will support 8 MiB blocks soon, it seems prudent to increase the default MaxTxBytes to something larger than 8 MiB so that the mempool has more capacity than a single block. If we don't update the default, we may end up in scenarios where a block gets produced with all the transactions from the mempool and then nothing is included in subsequent blocks because the mempool is empty. In order to action this issue we should:

1. Investigate the downsides of increasing this value
2. Investigate what other chains set this value to
3. Propose candidate values based on the above
4. Get alignment from team, then update the default
5. [Optional] ask Arabica node operators to modify the value in their local config

Related: #3928 and #2686 and #3966