Use `clippy::msrv` instead of `allow(clippy::incompatible_msrv)` when possible

The main clippy attribute for controlling the target MSRV is the `clippy::msrv` attribute. This is used by many lints to either improve the suggestion made or expand the cases that can be linted. Support for this was added in `1.50` which predates the `incompatible_msrv` lint by quite a bit.

The attribute has the form `#[clippy = "1.x.y"]` with the patch version being optional. It should function properly anywhere attributes are supported.