Bump the MSRV to 1.60

[tatsuya6502]

Bump the Minimum Supported Rust Versions (MSRVs) for all crate features from 1.51 to 1.60.

• Rust 1.51 was released on March 25, 2021.
• Rust 1.60 was released on April 7, 2022, and supports Rust 2021 Edition.

In order to address #243, quanta needs to be upgraded to v0.11.0. And quanta v0.11.0's MSRV is 1.60 (#251 (comment)).

We will release this moka version as v0.10.2, as we consider it is not a semver-breaking change:

https://github.com/moka-rs/moka#minimum-supported-rust-versions

It will keep a rolling MSRV policy of at least 6 months. If only the default features are enabled, MSRV will be updated conservatively. When using other features, like future, MSRV might be updated more frequently, up to the latest stable. In both cases, increasing MSRV is not considered a semver-breaking change.