Issue #638: Fix all our tests

[justafish]

• Replace sserbin/twig-linter with vincentlanglet/twig-cs-fixer/ - sserbin/twig-linter depends on an old version of symfony/console and core is adopting this new package at some point https://www.drupal.org/project/drupal/issues/3284817 - it also has a much better extension mechanism which will help if something like #615 comes up again
• Update Pantheon settings for Drupal 11
• Update PHPUnit configurations and tests for Drupal 11
• Fix mysterious yarn issues with DDEV