Use stable rust for cargo-fmt

[MarcoIeni]

Is there an existing issue or pull request for this?

• I have searched the existing issues and pull requests

Feature description

Not really a feature.
working with cargo nightly for cargo fmt is painful :(
Please, consider switching to stable to make the experience of contributors a bit better.

Desired solution

• Remove rust-toolchain.toml file
• Run cargo fmt

Downside: all opened PRs will have merge conflicts probably because stable uses spaces instead of tabs. So it's better to do this when there aren't big PRs opened.

Alternatives considered

The alternative is keeping the current configuration I guess.

Additional context

feedback coming from #613 (comment)