[Mailer] fix deps=low

Author: nicolas-grekas

composer.json

@@ -20,7 +20,7 @@
         "egulias/email-validator": "^2.1.10",
         "psr/log": "~1.0",
         "symfony/event-dispatcher": "^4.3",
-        "symfony/mime": "^4.3.3|^5.0",
+        "symfony/mime": "^4.4|^5.0",
         "symfony/service-contracts": "^1.1"
     },
     "require-dev": {