Stop suggesting legacy utilities (#19169)

It was already supposed to work this way — and it appeared via the tests
that it did — but the tests weren't loading the design system normally
so when we split the legacy utilities into a separate file it stopped
testing this properly.

The setup here is a bit iffy but the gist is:
- We allow static utilities to be suggested by default
- A static utility can *opt out* of suggestions by explicitly
registering themselves to return none.