Create enums for RequirementSeverity and deprecate drupal_requirements_severity() constants

Created a MR, but I know there will be test fails. I think the biggest issue is going to be BC support and converting from int values stored in config.

Remaining tasks:

• Handle BC for contrib modules that will still be using constants.

I think this issue is big enough without trying to introduce enums for RequirementPhase at the same time.

Also adding that we are deprecating drupal_requirements_severity() and moving this to the enum.

Can we add a simple deprecation test to make sure the current constants are returning a deprecation warning.

Yeah, I think we need tests to make sure the BC layer works when using those constants.

Added a bc test for getMaxSeverity(). There are a few instances where we check the severity status.

BC tests added look great! Moving to NW for the CR.

Updated CR

Thanks! CR reads well.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

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Rebase on 11.x

Posted a lengthy review.

    There was 1 error:
    
    1)
    Drupal\Tests\system\Functional\UpdateSystem\UpdateScriptTest::testRequirements
    Behat\Mink\Exception\ResponseTextException: The text "This is a
    requirements warning provided by the update_script_test module." was not
    found anywhere in the text of the current page.
    
    /builds/issue/drupal-3410938/vendor/behat/mink/src/WebAssert.php:907
    /builds/issue/drupal-3410938/vendor/behat/mink/src/WebAssert.php:293
    /builds/issue/drupal-3410938/core/tests/Drupal/Tests/WebAssert.php:956
    /builds/issue/drupal-3410938/core/modules/system/tests/src/Functional/UpdateSystem/UpdateScriptTest.php:150
    /builds/issue/drupal-3410938/vendor/phpunit/phpunit/src/Framework/TestResult.php:728

Added further feedback, tests still failing.