Remove support for Protobuf 2.x and protobuf-java-format

Protobuf 3.x is now well established in the community, so we should raise the minimum baseline for the 6.1 generation.

Also, our integration with `"com.googlecode.protobuf-java-format:protobuf-java-format"` predates the official JSON support from Protobuf Java. This library hasn't seen any update in years now, it should be safe to remove the support entirely now.