Bug #118868 Make the use_gr_notifications option enabled by default to cut Router's metadata handling overhead

Description:
Thanks to https://dev.mysql.com/worklog/task/?id=10719, we have a great new functionality allowing switching from TTL-based metadata cache refresh to event-based notifications using X protocol listener. 

The old behavior brings a lot of unnecessary overhead in terms of binlog writes to the `mysql_innodb_cluster_metadata`.`routers` table, as, regardless of whether there is any change or not, it always executes UPDATE queries via the ClusterMetadata::update_router_last_check_in function:
https://github.com/mysql/mysql-server/blob/8.4/router/src/metadata_cache/src/cluster_metad...
 With ROW-based replication, this puts the whole row twice each time into the binary log event.
Now, the rows in version 8.4 are many times bigger due to a major change in storing the whole configuration:
https://dev.mysql.com/doc/mysql-router/8.4/en/mysql-router-configuration-file-locations.ht...

With the router's use_gr_notifications=1 option, the overhead drops to an absolute minimum as the metadata table is updated only as a result of cluster changes. I don't see any reason why this behavior should not be the default. The TTL-based refresh is still active as a fallback in the case of X protocol listener malfunction. If there are any potential drawbacks, please add them to the documentation.

How to repeat:
Add more 8.4 routers to an idle InnoDB Cluster and observe how much binlog writes amplify!

Suggested fix:
Make use_gr_notifications enabled by default to minimize the write overhead.

Hello Przemyslaw,

Thank you for the reasonable feature request!

Thanks,
Umesh

I presented some example ramifications of this in the following post:
https://www.percona.com/blog/mysql-router-8-4-how-to-deal-with-metadata-updates-overhead/