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 183fb200ce
PiperOrigin-RevId: 721207574

java/core/src/main/java/com/google/protobuf/RepeatedFieldBuilderV3.java

@@ -25,6 +25,7 @@ import java.util.RandomAccess;
  *     should update gencode to >= 4.26.x which replaces RepeatedFieldBuilderV3 with
  *     RepeatedFieldBuilder.
  */
+@Deprecated
 public class RepeatedFieldBuilderV3<
         MType extends AbstractMessage,
         BType extends AbstractMessage.Builder,