The prior workflow (now deleted) required manually specifying the packages to
check and only checked the package could compile under the stated MSRV. It
didn't verify it was actually the _minimum_ supported Rust version. The new
version finds the MSRV from scratch to check if the stated MSRV aligns.
Updates stated MSRVs accordingly.
Also removes many explicit dependencies from secq256k1 for their re-exports via
k256. Not directly relevant, just part of tidying up all the `toml`s.
This will effectively add msrv protections to the entire project as almost
everything grabs from these.
Doesn't add msrv to coins as coins/bitcoin is still frozen.
Doesn't add msrv to services since cargo msrv doesn't play nice with anything
importing the runtime.
