Merge branch '4.4' into 5.2

* 4.4:
  Backport psr/container 1.1/2.0 compatibility
  Bump Symfony version to 4.4.21
  Update VERSION for 4.4.20
  Update CONTRIBUTORS for 4.4.20
  Update CHANGELOG for 4.4.20

Author: derrabus

composer.json

@@ -23,7 +23,7 @@
         "symfony/deprecation-contracts": "^2.1"
     },
     "require-dev": {
-        "psr/container": "^1.0",
+        "psr/container": "^1.0|^2.0",
         "symfony/event-dispatcher": "^4.4|^5.0",
         "symfony/expression-language": "^4.4|^5.0",
         "symfony/http-foundation": "^4.4|^5.0",