[ObjC] Scrub "syntax" references and editions prep

Scrub all the objc related sources for references to "syntax"
and update them to no longer tie things accordingly.

The ObjC Protos `GPBFileDescriptor` concept still exposes the
`syntax`, but it was already marked as deprecated. This will be
removed in the future as nothing should need to look at the value.

Validate that all generation calls are on the helpers to get the
editions support from the C++ code.

PiperOrigin-RevId: 555971288