Use PROTOBUF_DEPRECATED instead of PROTOBUF_DEPRECATED_ENUM (#12026)

At the time the way of marking some stuff `deprecated` is overcomplicated taken into account that protobuf mandates C++11 to be built.

Particularly, PROTOBUF_DEPRECATED_ENUM references gcc-6 release notes and the compiler is almost 4.5 years old as of today.
I believe it should be possible to simplify the definitions.

Closes #12026

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/12026 from georgthegreat:deprecated 536c3c9ff70c5c4bbd845820fc6cd8a5e4e7a2bf
PiperOrigin-RevId: 514480030