Use pre-parsed index mode when figuring out timestamp range (#88254)

Use `indexMode` field instead of parsing IndexMode from settings in
order to determine time series timestamp range. Parsing the index mode
isn't necessary, because we read the index mode when IndexMetadata is
constructed, and this just burns unnecessary CPU cycles.

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

@@ -1006,7 +1006,11 @@ public IndexLongFieldRange getTimestampRange() {
      */
     @Nullable
     public IndexLongFieldRange getTimeSeriesTimestampRange() {
-        return IndexSettings.MODE.get(settings).getConfiguredTimestampRange(this);
+        if (indexMode != null) {
+            return indexMode.getConfiguredTimestampRange(this);
+        } else {
+            return null;
+        }
     }
 
     @Override