rust: bump Clippy's MSRV and clean `incompatible_msrv` allows

rust: bump Clippy's MSRV and clean `incompatible_msrv` allows

Following the Rust compiler bump, we can now update Clippy's MSRV we
set in the configuration, which will improve the diagnostics it generates.

Thus do so and clean a few of the `allow`s that are not needed anymore.

Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-7-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>