Add rust-version field to all Cargo.toml files

[xtqqczze]

This is necessary to prevent Clippy from producing incorrect suggestions.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tail/truncate. tests/tail/truncate is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/tail/retry is no longer failing!

[cakebaker]

Why not set msrv in .clippy.toml? For some reason it got removed with the recent update of the MSRV.

[oech3]

I don't think .clippy.toml work as @cakebaker assumed. clippy still produced a bunch of wanings before this discussion.

[oech3]

Does not this PR remove clippy::manual_is_multiple_of config?

[xtqqczze]

Why not set msrv in .clippy.toml? For some reason it got removed with the recent update of the MSRV.

The incorrect suggestions are in fact caused by the removal of msrv from .clippy.toml.

I initially thought this was no longer necessary because we specify rust-version in Cargo.toml, but that setting wasn’t applied at the workspace level as I had assumed.

Even so, I still think this is the better approach.

[xtqqczze]

Does not this PR remove clippy::manual_is_multiple_of config?

manual_is_multiple_of isn't affected; it is correct for clippy to make the suggestions because unsigned_is_multiple_of is a 1.87 feature.

[cakebaker]

Thanks!