Drupal 9 Compatibility

Looks good to me @scott_euser . As `core: 8.x` in info.yml file still allows to install this on version prior 8.7.7 I was wondering if the `$defaultTheme` variable could cause any issue in there, but I believe that change will be just ignored and so all good. RTBC.

Drupal\Tests\BrowserTestBase::$defaultTheme is required in drupal:9.0.0 when using an install profile that does not set a default theme. See https://www.drupal.org/node/3083055, which includes recommendations on which theme to use.

Existing tests which the developer doesn't want to update assertions for
Add:
  protected $defaultTheme = 'classy';
RECOMMENDED Existing tests which the developer wishes to update assertions for to minimize future change
For tests relying on no markup at all or at least no core markup (⚠️This is strongly recommended: avoiding coupling tests to markup whenever possible makes those tests more robust! ⚠️):
  protected $defaultTheme = 'stark';
For tests relying on core markup:

  protected $defaultTheme = 'stable';
New tests
For tests relying on no markup at all or at least no core markup:
  protected $defaultTheme = 'stark';
For tests relying on core markup:

  protected $defaultTheme = 'stable';

Adding protected $defaultTheme = 'classy'; is the quickiest way to fix warnings without update assertions. But it might be good to change it to stark or stable. Or out of this issue's scope?

Yep, the change makes sense.

Closed #3068107: Replace usage of entity.manager with entity_type.manager in Drupal\role_delegation\Plugin\views\field\RoleDelegationUserBulkForm as a duplicate.

Committed and pushed to 8.x-1.x. Thanks!