Remove experimental flags

Migrated issue

Reported by: fjgarlin

Related to !142 (merged)

Problem/Motivation

There are plenty of Drupal 7 projects already integrated in GitLab CI, and there are plenty of Drupal 10+ projects using the _PHPUNIT_CONCURRENT flag (see here https://git.drupalcode.org/search?group_id=2&repository_ref=main&scope=blobs&search=_PHPUNIT_CONCURRENT)

These two things were marked as experimental long ago when they were introduced but that is no longer the case.