[ticket/12008] The read_notification_last_gc config setting should be dynamic.

PHPBB3-12008
[ticket/12008] Update the run time value for the prune notifications cron task

PHPBB3-12008
Merge remote-tracking branch 'prototech/ticket/12008' into develop

* prototech/ticket/12008:
  [ticket/12008] Update the run time value for the prune notifications cron task
  [ticket/12008] The read_notification_last_gc config setting should be dynamic.