Set MSRV to latest stable

[thejpster]

We're fighting a losing battle with our dependencies because they keep needing a newer Rust than our MSRV.

I propose we just set the MSRV to latest stable and be done with it. And at the same time, set any clippy jobs to not "Deny all Warnings" because new warnings come along all the time and I don't a new warning to break the build (but I do want to know about them so I can choose to fix them if required).

[jannic]

Isn't the dependency issue mostly solved with this change?

In my opinion, MSRV should only be increased when it is needed to get some relevant advantage.
That could be a new feature we want to use. Either in the language itself, or in some updated dependency that needs the newer Rust version.

Of course, the old situation where CI broke every now and then because some dependency decided to bump the MSRV in a minor version was very unsatisfying. This should be solved by the new resolver mode. Sure, we may learn that it doesn't work as well in practice (e.g. when crates we depend on start to require newer Rust versions implicitly, without adjusting the declared MSRV). But I'd prefer to actually try that out and see how well it works.

Regarding the clippy warnings: Here I don't care too much. We should strive for a warning-free build either way, but I agree that it is really annoying when a new clippy lint breaks the build. Locally, I use the beta compiler by default, so I tend to notice new warnings before they reach our CI and can send fixes proactively. (E.g. #992)
I looked at all the failed pipeline runs in https://github.com/rp-rs/rp-hal/actions?query=is%3Afailure where the logs are still available, and I didn't see a single case where a new lint broke the build. Is this really a problem we need to solve?

[thejpster]

Ok, the new resolver might be enough. Happy to try it out for a while!