Fix CI failure

[yutannihilation]

This pull request does:

1. Drop the CI for R 4.1
2. Add a CI for R-devel
3. Drop 32bit-related setup from the CI setup and README
4. Use dtolnay/rust-toolchain action instead of unmaintained action-rs/toolchain action

For point 1, I think we don't care much about the backward-compatibility with that old version. That said, it should be good to test against multiple R versions, so I included point 2, but this is optional.

[yutannihilation]

I'll merge this in a day or two, but I'm keen to hear comments on the removing the Rust version specification.