Annotate RepeatedFieldBuilderV3 with @Deprecated. (#20030)

This silences a javac lint:
```
java/core/src/main/java/com/google/protobuf/RepeatedFieldBuilderV3.java:28: warning: [dep-ann] deprecated item is not annotated with @Deprecated
public class RepeatedFieldBuilderV3<
       ^
```

Closes #20030

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/20030 from benjaminp:dep-ann 183fb200ce62059b94f53e06ce77f430eb0bdaf0
PiperOrigin-RevId: 721207574