Modernize CI to use current, non-archived actions

[trappitsch]

As discussed in #59, this PR updates the CI, which is now using to the following:

• Use actions/checkout@v4
• For rust toolchain use dtonlay/rust-toolchain@stable

The rust version is pinned with an env variable named MSRV to 1.76. This applies for all builds, tests, fmt, and clippy.

On target x86_64-unknown-linux-gnu, all features are built and tested. I also added a matrix entry for the regex feature alone, as this has previously only been tested as part of the from_str feature. Adding regex alone as well seems more consistent, as we also test std alone (even though it is part of from_str as well). Also continuing to test nightly but don't fail.

On targets thumbv6m-none-eabi and thumbv7m-none-eabi, as previously, the cargo build is run. Continue to do the same with nightly but don't fail.

cargo fmt is still here as before, so is cargo clippy. The latter again is also matrixed with all features as for the x86_64... target.

[thejpster]

Perfect. No notes.

[trappitsch]

Thank you!