Use check-cfg lint config instead of allowing unexpected_cfgs

[Urgau]

With the release of rust-lang/cargo#13913 (in nightly-2024-05-19), Cargo has now gain the ability to declare --check-cfg args directly inside the [lints] table with [lints.rust.unexpected_cfgs.check-cfg]¹:

Cargo.toml:

[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(loom, msrv)'] }

This PR therefore just replace the global #![allow(unexpected_cfgs)] with the above.

Footnotes

1. take effect on Rust 1.80 (current nightly), is ignored on Rust 1.79 (current beta), and produce an unused warning below ↩