readme: update clippy version in contrib guide (#5623)

In CI, we are using a newer version of Clippy than what is stated in the
contributions guide.

Additionally, it is no longer necessary to use Clippy from the MSRV. As
of Clippy 1.64, the `rust-version` field in Cargo.toml is respected.

The text and the command have been updated to reflect the current state
of CI and best practices.

CONTRIBUTING.md

@@ -131,8 +131,11 @@ cargo check --all-features
 cargo test --all-features
 ```
 
-Clippy must be run using the MSRV, so Tokio can avoid having to `#[allow]` new
-lints whose fixes would be incompatible with the current MSRV:
+Ideally, you should use the same version of clippy as the one used in CI
+(defined by `env.rust_clippy` in [ci.yml][ci.yml]), because newer versions
+might have new lints:
+
+[ci.yml]: .github/workflows/ci.yml
 
 <!--
 When updating this, also update:
@@ -146,7 +149,7 @@ When updating this, also update:
 -->
 
 ```
-cargo +1.49.0 clippy --all --tests --all-features
+cargo +1.65.0 clippy --all --tests --all-features
 ```
 
 When building documentation normally, the markers that list the features