Update notification for additional plugins doesn't work anymore

1. Install an older version of a plugin available at the Plugins directory
2. Let the cron run the task Check for updates (\core\task\check_for_updates_task)
3. TEST: Make sure the site admin receive the notification about available update for the outdated plugin via e-mail (there are messaging settings for the actual target of the notification, e-mail should be enabled by default).
   Master branch will not receive any updates because this version is not registered in plugins db

Notes:

• You may wish to modify the scheduled task to run more often than default every two hours while testing this.
• The code won't fetch new updates info if the recently fetched data are considered fresh enough. You may wish to UPDATE the value 'recentfetch' of the component 'core_plugin' in the 'config_plugins' table to a low value to force your Moodle to re-fetch data. Make sure you purge caches after manually setting the data in the config_plugins table.