Cron URL (route) cannot be executed while in maintenance mode

Problem/Motivation

Right now (10.6.x and i assume 11.x) allows to run cron through the "run cron" UI button when the site is with the maintenance mode enabled or disabled.
However, cron can also be run through one certain url (/cron/{key}) defined in system.cron on core/modules/system/system.routing.yml
If it is needed to run cron using the mentioned URL under certain conditions (eg: a cronjob that can only call the url and not use drush cron instead), the operation fails if the site is under maintenance mode (and therefore the cron can only be executed manually)
This can cause sites under maintenance not fully operate if they still need to execute certain cron tasks and relying on the server cronjob, as the url will not be reachable, as the system is designed like that (quoting @berdir on slack:

there is no cron.php anymore, it's a regular route in D8+, and a regular route is subject to the maintenance request subscriber unless that has explicit rules on excluding something. most likely, nobody thought about that during the conversion.

)

Steps to reproduce

- call from a browser the cron url -> cron is executed
- set maintenance mode on
- call the same url again -> cron is not executed
- trigger cron from the UI as admin -> cron is executed

Proposed resolution

The mentioned defined route has one custom access_check set in \Drupal\system\Access\CronAccessCheck that is only used there.
The CronAccessCheck class has the following piece of code that is what is blocking cron execute it during maintenance mode:

    elseif (\Drupal::state()->get('system.maintenance_mode')) {
      \Drupal::logger('cron')->notice('Cron could not run because the site is in maintenance mode.');
      return AccessResult::forbidden()->setCacheMaxAge(0);
    }

in addition, the route needs to allow being called during maintenance mode, so the next change is also needed:

        _maintenance_access: TRUE

under options: in the system.cron route (system.routing.yml)

After those changes, the cron url is allowed to be executed also during cron

The first change is valid for 11.x and 10.6.x but the second differs between versions, as the route is located in different lines of the same file (so backporting is not automatic)

Many thnks in advance