Turn off show_alerts_predefined_layouts by default (#26242)

Author: maxiadlovskii

graylog2-server/src/main/resources/org/graylog2/featureflag/feature-flag.config

@@ -131,4 +131,4 @@ asset_right_sidebar=on
 replay_search_right_sidebar=on
 
 # Show predefined table layouts for alerts data table
-show_alerts_predefined_layouts=on
+show_alerts_predefined_layouts=off