Switch interval validation to accept DateHistogramInterval

Author: gmarouli

server/src/main/java/org/elasticsearch/cluster/metadata/DataStreamLifecycle.java

@@ -576,7 +576,10 @@ static void validateRounds(List<DownsamplingRound> rounds) {
                                 + "."
                         );
                     }
-                    DownsampleConfig.validateSourceAndTargetIntervals(previous.config(), round.config());
+                    DownsampleConfig.validateSourceAndTargetIntervals(
+                        previous.config().getFixedInterval(),
+                        round.config().getFixedInterval()
+                    );
                 }
             }
         }