Merge branch '6.3' into 6.4

* 6.3:
  [6.3] Remove unused test fixture
  [5.4] Remove unused test fixtures
  [Dotent] Add PHPDoc for `$overrideExistingVars`
  [SecurityBundle] Fix missing login-link element in xsd schema
  [Validator] Add missing Chinese translations #51934
  [Serializer] Fix using `DateIntervalNormalizer` with union types
  [Validator] fix: add missing translations for for Thai (th)
  fix #52273 [doctrine-messenger] DB table locks on messenger_messages with many failures
  [Serializer] Handle defaultContext for DateTimeNormalizer
  declare constructor argument as optional for backwards compatibility
  [CI] Add step to verify symfony/deprecation-contracts requirements

Author: fabpot