Merge branch '6.4' into 7.0

* 6.4:
  Sync .github/expected-missing-return-types.diff
  [WebProfilerBundle] Fix the design of the compact toolbar button
  [AssetMapper] Improve link generation script
  fix merge
  consistently use dates in UTC in Doctrine connection tests
  Add missing Chinese(zh_TW) translations
  [Intl] [Emoji] Fix emoji files (remove wrong characters / add missing data)
  [Translation] Crowdin Bridge: use the project language mapping
  [VarExporter] Nitpicking in README.md
  [DoctrineBridge] Adapt deprecation message to include ODM bundle attribute name
  Added missing Serbian (sr_Cyrl) translation
  [Validator] Add missing Serbian translation
  [Validator] Add missing arabic translation
  [Validator] updated Albanian translation
  [Clock] Fix calling `mockTime()` in `setUpBeforeClass()`

Author: nicolas-grekas