fix default value for elasticsearch.requestTimeout

make the value match the es client default and the joi config value

Fixes #6377

config/kibana.yml

@@ -54,7 +54,7 @@
 
 # Time in milliseconds to wait for responses from the back end or elasticsearch.
 # This must be > 0
-# elasticsearch.requestTimeout: 300000
+# elasticsearch.requestTimeout: 30000
 
 # Time in milliseconds for Elasticsearch to wait for responses from shards.
 # Set to 0 to disable.