Rust 1.56.0 pre-release testing

[steffahn]

I’ll quote this from the last pre-release testing thread:

@josh

Also, I think we may want to change the recommended test procedure for pre-releases. Rather than having people do an early upgrade of their stable toolchain, I think we should have people do rustup toolchain add 1.55.0 and then test with cargo +1.55.0 or similar. That way, they still have their normal stable toolchain until the actual stable release, and if they forget to update again right away, they don't end up continuing to run the pre-release for a while.