Ignore cron when maintenance mode is enabled

This MR is running on prod without problem since January, to me it could be marked as RBTC

Was dealing with the same but needed it for mutliple drush tasks for other modules as well, this might help: https://www.drupal.org/project/drush_firewall →

For the record, drush now suggests → to add a condition using drush maint:statusin the cron line:

10 * * * * cd [DOCROOT] && /usr/bin/env PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin COLUMNS=72 ../vendor/bin/drush --uri=your.drupalsite.org --quiet maint:status && /vendor/bin/drush --uri=your.drupalsite.org --quiet cron

Is there a use-case where people would want to run cron in maintenance mode though? E.g. maybe doing maintenance mode pre-live as a sort of holding page. I think we should make this a checkbox in the configuration with:

- Update hook to make it disabled for existing sites
- Default config install to enable it for new sites

Thanks!