Auto merge of #41751 - alexcrichton:unstable-flags, r=eddyb

Auto merge of #41751 - alexcrichton:unstable-flags, r=eddyb

rustc: Forbid `-Z` flags on stable/beta channels

First deprecated in rustc 1.8.0 the intention was to never allow `-Z` flags make
their way to the stable channel (or unstable options). After a year of warnings
we've seen one of the main use cases, `-Z no-trans`, stabilized as `cargo
check`. Otherwise while other use cases remain the sentiment is that now's the
time to start forbidding `-Z` by default on stable/beta.

Closes #31847