Merge branch '5.1' into 5.x

* 5.1:
  fix merge
  fix merge
  Remove branch-version (keep them for contracts only)
  [HttpClient] relax auth bearer format requirements
  [PHPUnitBridge] Silence errors from mkdir()
  [DependencyInjection] Preload classes with union types correctly.
  [Serializer] fix decoding float XML attributes starting with 0
  add missing dutch translations
  [TwigBridge] Remove "transchoice" from the code base
  Support PHPUnit 8 and PHPUnit 9 in constraint compatibility trait
  Add expectDeprecation, expectNotice, expectWarning, and expectError to TestCase polyfill
  [String] fix before/after[Last]() returning the empty string instead of the original one on non-match
  Add missing exporter function for PHPUnit 7
  [Validator] Add missing romanian translations
  [String] fix slicing in UnicodeString
  [Cache] Use correct expiry in ChainAdapter
  do not translate null placeholders or titles

Author: nicolas-grekas

composer.json

@@ -27,8 +27,5 @@
             "/Tests/"
         ]
     },
-    "minimum-stability": "dev",
-    "extra": {
-        "branch-version": "5.2"
-    }
+    "minimum-stability": "dev"
 }