Add deprecation notice to GetOwner and SetOwner for groups

This commit puts warnings and notes in place so that users become aware
that GetOwner and SetOwner will be deprecated in a future Gerrit release
once we are fully on NoteDb for groups. It on purpose does not speak
about alternatives as these should be added once groups for NoteDb is
the default.

Change-Id: I0e348d13dd6b007a23661f0431d29780d3795204

java/com/google/gerrit/extensions/api/groups/GroupApi.java

@@ -40,7 +40,12 @@ public interface GroupApi {
    */
   void name(String name) throws RestApiException;
 
-  /** @return owning group info. */
+  /**
+   * @return owning group info.
+   * @deprecated for groups in NoteDb, use READ/PUSH permissions on the group ref to express
+   *     ownership.
+   */
+  @Deprecated
   GroupInfo owner() throws RestApiException;
 
   /**
@@ -48,7 +53,10 @@ public interface GroupApi {
    *
    * @param owner identifier of new group owner.
    * @throws RestApiException
+   * @deprecated for groups in NoteDb, use READ/PUSH permissions on the group ref to express
+   *     ownership.
    */
+  @Deprecated
   void owner(String owner) throws RestApiException;
 
   /** @return group description. */