Add stubs for GeneratedMessageV3, RepeatedFieldBuilderV3, SingleFieldBuilderV3 for compatibility with older <4.26.x gencode.

These classes are deprecated and will be removed in the next breaking change. Users should update gencode to >= 4.26.x which uses GeneratedMessage instead of GeneratedMessageV3.

Tested with //compatibility:java_conformance_v3.25.0 which builds the runtime against 3.25.0 gencode.

PiperOrigin-RevId: 644136172