[Mailer] fix lowest allowed dependencies

Author: xabbuh

composer.json

@@ -17,7 +17,7 @@
     ],
     "require": {
         "php": ">=7.1.3",
-        "symfony/mailer": "^4.4|^5.0"
+        "symfony/mailer": "^4.4.21|^5.2.6"
     },
     "require-dev": {
         "symfony/http-client": "^4.3|^5.0"