Migrate syntax reflection to new feature-based system.

As part of go/protobuf-editions, we will be abandoning `syntax` in place of the new editions framework.  The first edition will turn each difference between proto2 and proto3 into a more targeted *feature* placed on proto descriptors.  In preparation for this, we want to migrate everyone off direct inspection of `syntax`.

PiperOrigin-RevId: 515457103