fallback on 'migrate' as the database key causes error messages if migrations use a different source

I get the above error message when I do a drush cr followed by drush ms if I do drush ms again there is no error message until I do another drush cr

Drupal 10.3.10

Migrate Plus 6.0.5
Migrate Tools 6.0.5

The error is originating from migrate_drupal_migration_plugins_alter() but should not be emitted. The code is doing a requirements check for migrating the "variables" subsystem, but this doesn't exist in Drupal 8+ since variables were replaced with config.

There is similar code just above that will suppress the exception for sites running Drupal 7+ which do not meet the requirements of the D6 taxonomy vocabulary migration:

    catch (RequirementsException $e) {
      // This code currently runs whenever the definitions are being loaded and
      // if you have a Drupal 7 source site then the requirements will not be
      // met for the d6_taxonomy_vocabulary migration.
    }

We can follow the same pattern, something like this:

    catch (RequirementsException $e) {
      // This code currently runs whenever the definitions are being loaded and
      // if you have a Drupal 8+ source site then the requirements will not be
      // met for the system_site migration since the 'variables' subsystem was
      // replaced with config entities in Drupal 8.
    }