Implicit quantification in type synonyms: enable warning by default, add test for #24090

This MR:

• adds a test for #24090 (closed), which was fixed in 0d4ee209,
• turns the implicit-rhs-quantification warning flag on by default, and adjusts the error message to make it clearer it will become an error very soon

The next step is #25911, which should be tackled for GHC 9.16 (or perhaps 9.18).