Drupal 9 compatibility

Problem/Motivation

We found ourselves needing this module to be Drupal 9 compatible, Drupal 8's eol is near November 2nd 2021.
We run Update Status to see the minimum needed to support that:

FILE: modules/contrib/micro_sso/tests/src/Functional/LoadTest.php
 
STATUS         LINE                           MESSAGE
--------------------------------------------------------------------------------
Check manually 13   Drupal\Tests\BrowserTestBase::$defaultTheme is required. See
                    https://www.drupal.org/node/3083055, which includes
                    recommendations on which theme to use.
--------------------------------------------------------------------------------
 
FILE: modules/contrib/micro_sso/micro_sso.info.yml
 
STATUS         LINE                           MESSAGE
--------------------------------------------------------------------------------
Check manually 0    Add core_version_requirement: ^8 || ^9 to designate that the
                    module is compatible with Drupal 9. See
                    https://drupal.org/node/3070687.
--------------------------------------------------------------------------------

Proposed resolution

Create a patch containing the above recommendations.

I'll try to do it soon!