Replace deprecated REQUIREMENT_ERROR constant in search_api_db_defaults_requirements() for Drupal 11+ compatibility

All tests passed (PHPUnit, PHPCS, PHPStan), but the pipeline shows as failed due to a post-job exit code 1.
The patch simply replaces the deprecated REQUIREMENT_ERROR with RequirementSeverity::Error, ensuring compatibility with Drupal 11.2+ and 12.x. I think it should be safe to merge it. Moving to Needs review.

drunken monkey → made their first commit to this issue’s fork.

I rewrote the code to use DeprecationHelper, which I think is the “proper” way to do this. I also temporarily enabled testing against other Drupal versions to make sure this is compatible with all versions we want to support.
However, tests are still failing and I have no idea, why.

Since this is part of fixing the pipelines, I’m integrating this fix into 🐛 Fix failing pipelines Active . Maybe an MR with all fixes will help resolve the failure in the MR?

Thanks a lot for reviewing it. Honestly I don't know either why test are failing. Summary shows that the test are passing:
https://git.drupalcode.org/project/search_api/-/pipelines/563625/test_re...

But job is failing, I am looking into SYMFONY_DEPRECATIONS_HELPER and these two issues (I am not familiar with it)
🐛 [GitlabCI] run-tests.sh does not pass on SYMFONY_DEPRECATIONS_HELPER environment variable value to spawned processes Fixed
#3400979: Is SYMFONY_DEPRECATIONS_HELPER: weak correct for contrib deprecation testing needs? →
Maybe deprecation warnings are making the job to fail?