Issue #3556305: Convert plugin annotations to PHP 8 attributes

Kim Pepperrequested to merge
issue/purge-3556305:3556305-convert-attributes into 8.x-3.x
  • Created 7 new attribute classes in src/Attribute/
  • Updated all 7 plugin managers to support both attributes and annotations
  • Converted all plugins to use attributes instead of annotations
  • Updated PluginManagerTest classes to handle optional fields (deriver)
  • Added #[RunTestsInSeparateProcesses] to all Kernel test classes for PHPUnit 12 compatibility
  • Fixed missing return type on CacheableResponseSubscriber::getSubscribedEvents()

Closes #3556305

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