RFC 2495 (rust-lang/rfcs#2495) was merged
quite a while ago, but I think it was never actually implemented.

feat: Add `-Zmsrv-policy` feature flag

### What does this PR try to resolve?

Nothing noticeable....

The intent is to unblock experiments with different compatible MSRV policies like
- #9930
- #10653
- #10903

While I normally don't like PRs that do nothing on their own, this at least allows any one of those efforts to move forward with different people without juggling these base commits for whoever is first to include in their PR

While there isn't an RFC for this yet, this is intended to allow us to experiment to get a better idea of what we should put in an RFC.  In some cases, we first do an eRFC for this but I assumed this wouldn't be needed in this case as this builds on rust-lang/rfcs#2495 and, I'm assuming, will be more surgical in nature

### How should we test and review this PR?

The `Summary` changes are largely untested as they will be mostly tested through the future work that builds on this PR.  However, I wasn't too concerned about that because the code is relatively trivial.

### Additional information

I chose the name `msrv-policy` to distinguish this unstable feature from `rust-version`.  Though those appear in different places (`Cargo.toml` vs `-Z`), I can see them being confusing which was especially apparent when editing `unstable.md` which has an anchor for `rust-version`.