Merge branch '6.0' into 6.1

* 6.0:
  Update ComposerPlugin.php
  [Notifier] [OvhCloud] handle invalid receiver
  [Cache] fix collecting cache stats when nesting computations
  [VarDumper] Fix JS to expand / collapse
  [Validator] Fix Email validator logic
  Fix user_identifier support after username has been deprecated in favor of it.
  [Tests] Remove `$this` occurrences in future static data providers
  [PropertyInfo] Fixes constructor extractor for mixed type
  use method_exists() instead of catching reflection exceptions

Tests/EventListener/ErrorListenerTest.php

@@ -207,7 +207,7 @@ public function controllerProvider()
         }];
 
         yield [function ($exception) {
-            $this->assertInstanceOf(FlattenException::class, $exception);
+            static::assertInstanceOf(FlattenException::class, $exception);
 
             return new Response('OK: '.$exception->getMessage());
         }];