Merge branch '6.4' into 7.0

* 6.4: (32 commits)
  [Validator] Fix registering "is_valid()" for `#[Expression]`
  [Scheduler] Trigger unique messages at runtime
  [Scheduler] Fix CHANGELOG
  [Clock] Add `DatePoint`: an immutable DateTime implementation with stricter error handling and return types
  [Scheduler] Allow modifying the schedule at runtime and recalculate heap
  [Cache] Fix Redis6Proxy
  [Finder] Disable failing test about open_basedir
  Fix merge
  Fix merge
  Minor CS fixes
  Deprecate `Kernel::stripComments()`
  Remove setAccessible reflection call in tests
  [Notifier] Telegram Bridge add escaping for \
  [Component][AssertMapper] add type hint of an argument in asset mapper command
  [Translation] [Phrase] Refacto ReadConfig and WriteConfig into arrays
  [Routing] Fix routing collection defaults when adding a new route to a collection
  [Messenger] Fix cloned TraceableStack not unstacking the stack independently
  [Translation] Add `--as-tree` option  to `translation:pull` command
  [Mime] Allow to add some headers as a strings
  [Translation] Give current locale to locale switcher runWithLocale callback
  ...

Author: nicolas-grekas

LockFactory.php

@@ -13,7 +13,6 @@
 
 use Psr\Log\LoggerAwareInterface;
 use Psr\Log\LoggerAwareTrait;
-use Psr\Log\NullLogger;
 
 /**
  * Factory provides method to create locks.