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

[Jarcho]

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.