[Contracts] add return types and bump to v3

Author: nicolas-grekas

composer.json

@@ -20,8 +20,8 @@
         "psr/log": "~1.0"
     },
     "require-dev": {
-        "symfony/event-dispatcher-contracts": "^2",
-        "symfony/http-client-contracts": "^2",
+        "symfony/event-dispatcher-contracts": "^2.0|^3.0",
+        "symfony/http-client-contracts": "^2.0|^3.0",
         "symfony/messenger": "^5.4|^6.0"
     },
     "conflict": {