Update MSRV to current stable, 1.56.1

[notmandatory]

Description

Update MSRV to the current stable, 1.56.1 and update the cargo.lock file. Also removed the rust-toolchain file.

Notes to the reviewers

I removed the rust-toolchain file because I don't think we need to lock users into a particular version (or force them to override it), and this file seem to primarily be used for projects using a specific nightly release, see: https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file.

I also had to pin the fd-lock dependency because of a recent update to that crate to use the rustdoc::missing_doc_code_examples feature which is only supported by nightly :disappointed: .

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] I ran cargo fmt and cargo clippy before committing

[notmandatory]

Before this is merged we need to change the github expected checks from 1.53.0 to 1.56.1.

[notmandatory]

undoing the pinned fd-lock with #64