tee: Remove deprecated retain_mut dependency when MSRV is >= 1.61.0

[oliviacrain]

The retain_mut crate is a dependency of the uu_tee crate. The functionality provided by retain_mut was stabilized in the standard library earlier this year in the 1.61.0 release. As a result, the maintainer of retain_mut has marked the crate as deprecated and no longer maintains it.

We should remove the dependency and use the standard library's functionality in uu_tee crate once we bump the MSRV to 1.61.0 or later. I don't think this presents a pressing reason to raise the MSRV. This is just a reminder to remove the dependency when the time comes.

[jfinkels]

Related to #3555

[tertsdiepraam]

Yeah, we can probably do this after the next release, which is when we usually bump MSRV. Thanks for making an issue for it!

[cakebaker]

This ticket can be closed, it has been implemented in bf8e933 .

[tertsdiepraam]

Indeed! I forgot that we also had an issue for it, thanks!