Deprecated javadoc

Author: Ray Mattingly

hbase-protocol-shaded/src/main/protobuf/Backup.proto

@@ -54,7 +54,7 @@ message TableServerTimestamp {
 
 /**
  * Structure keeps relevant info for backup restore session
- * backup_root_dir is deprecated because we no longer store root dirs on disk HBASE-28882
+ * backup_root_dir was marked as deprecated in HBase 2.6.2, will be removed in 4.0.0.
  */
 message BackupImage {
   optional string backup_id = 1;