Remove preview gating for newly-added stable fixes (#9681)
## Summary
At present, our versioning policy forbids the addition of safe fixes to
stable rules outside of a minor release, so we've accumulated a bunch of
new fixes that are behind `--preview`, and can be ungated in v0.2.0.
To find these, I just grepped for `preview.is_enabled()` and identified
all such cases. I then audited the `preview_rules` test fixtures and
removed any tests that existed only to test this autofix behavior.
