Add php-cs-fixer & PHPCompatibility check to workflow

[sreichel]

Description (*)

• adds PHPCompatibility check
• adds PHP-CS-Fixer to workflow
  • fixes last errors, not found by phpcs
• updated PhpStan
  • updated to v1.9.2
  • removed experimental check and set default level to 5 (from 3 befor)
• updated complete workflow (to run a bit faster)

Fixed Issues (if relevant)

1. Closes Discussion: Consider using PHP CS Fixer and/or PHPStan #947

Related PR

1. See Chore: Add Captainhook & PHPCS to composer require-dev and sample captainhook config #1264
2. See Running a PHP code sniff against fresh install for PHP compatibility with PHP 7 shouldn't find errors. #1157
3. See Phpstan: workflow update #2707

Manual testing scenarios (*)

php vendor/bin/php-cs-fixer fix --config=.github/php-cs-fixer-ruleset.php <path-to-files>

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (all builds are green)
• Add yourself to contributors list

[github-actions]

Unit Test Results

1 files  ±0  1 suites  ±0   0s ⏱️ ±0s
0 tests ±0  0 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
7 runs  ±0  5 ✔️ ±0  2 💤 ±0  0 ❌ ±0 

Results for commit 7fe65ad. ± Comparison against base commit 879caab.