Bump MSRV to 1.64?

[tertsdiepraam]

With a new release tagged, we have the opportunity to bump the MSRV again.

With 1.61 we would get:

• retain_mut, which unblocks https://github.com/uutils/coreutils/issues/4190
• The Termination trait which might simplify the uumain proc macro.

With 1.62:

• #[derive(Default)] for enums

With 1.63:

• Scoped threads

With 1.64:

• Workspace inheritance, which will simplify the dependencies.
• os::unix::process::CommandExt::process_group

With 1.65:

• let ... else (though this still breaks rustfmt)

With 1.66

• ..=X ranges
• Duration::try_from_secs_{f32, f64}

I think workspace inheritance would be a big improvement for this project, so I would propose to bump directly to 1.64.

[sylvestre]

sure, let's do it :)