This adds cxxbridge-cmd to Cargo.lock per
https://github.com/dtolnay/cxx/issues/1407#issuecomment-2509136343
It adds an MSRV to `src/taskchampion-cpp/Cargo.toml` so that the
version of `Cargo.lock` is stil compatible with the MSRV.
It additionally adds a check of the Cargo metadata for all of the cxx*
versions agreeing, and for the MSRV's agreeing.