Merge branch '6.0' into 6.1

* 6.0:
  [Translation] Fix Crowdin documentation urls
  ignore missing keys when mapping DateTime objects to uninitialized arrays
  fix writes to static $kernel property
  [Notifier] Fix test logic
  [Validator] Hint that `egulias/email-validator` needs to be installed for strict mode as default config

Author: xabbuh

Test/KernelTestCase.php

@@ -70,7 +70,7 @@ protected static function bootKernel(array $options = []): KernelInterface
 
         $kernel = static::createKernel($options);
         $kernel->boot();
-        self::$kernel = $kernel;
+        static::$kernel = $kernel;
         static::$booted = true;
 
         return static::$kernel;