Make a `.rust-toolchain` file with our MSRV

It is frustrating to develop with different people, CI, etc using different versions of the rust toolchain. To make sure this doesn't happen when we don't want it to, we should add a .rust-toolchain file specifying our MSRV, to ensure that everyone develops against that.

(This issue brought to you by cargo clippy --all-features --all-targets -- -D warnings failing on main with 1.80 but passing with 1.83...)