[Contracts] add return types and bump to v3

Author: nicolas-grekas

composer.json

@@ -21,7 +21,7 @@
         "php": ">=7.1.3"
     },
     "require-dev": {
-        "symfony/deprecation-contracts": "^2.1",
+        "symfony/deprecation-contracts": "^2.1|^3.0",
         "symfony/error-handler": "^5.4|^6.0"
     },
     "suggest": {