use gradle 8.11

Author: wing328

.github/workflows/samples-kotlin-server-jdk21.yaml

@@ -13,7 +13,7 @@ on:
       - 'samples/server/petstore/kotlin-server-required-and-nullable-properties/**'
 
 env:
-  GRADLE_VERSION: '8.10'
+  GRADLE_VERSION: '8.11'
 
 jobs:
   build: