feat/modernize

[ralflang]

• feat: Drop dependency on horde/test and use PHPUnit 11 directly.
• fix: Made tests compatible with PHPUnit 12. Removed one test which was an unconditional skip for multiple years
• style: Apply php-cs-fixer
• Add rule for implicitly nullables
• fix: Implicitly nullable parameters are not explicitly nullable
• fix: Removed guards no longer necessary due to signatures or new PHP behaviour >= 8.2
• style: Turn annotations into attributes
• ci: Don't run phpstan on test dir in the mandatory step but only in the allowed-to-fail step

[ralflang]

@TDannhauer I don't strictly need a review here but this is quite an instructional PR on the chores we do

• Remove the old test pipeline from .github/workflows and replace it with the (still imperfect) version you see here
• Add / update / replace the helper files
• .gitignore should not lose old entries
• phpunit.xml.dist usually can be replaced
• phpunit.xml should become phpunit.xml.dist unless both exist.
• edit .horde.yml and remove PHP 7 support, add the helpers you see in this PR.
• Use the horde-components.phar tool to update composer.json file from .horde.yml file. You can always get the latest version from this URL https://github.com/horde/components/releases manually. Or use https://github.com/horde/horde-support/pull/2/files#r2079241718 to automate it
  run "horde-components composer"
• Update the phpunit tests
• Watch phpstan for trivial deprecations and type issues

It's not necessary to do everything in one PR. For example, multiple libraries now run phpstan twice. First pass with level 5 makes the pipeline fail. Second pass with level 9 just complains but does not make the pipeline fail.

CC @Schlue