update java version in build.gradle (#18640)

Author: wing328

modules/openapi-generator-gradle-plugin/build.gradle

@@ -26,8 +26,8 @@ specifications as part of your build. Other tasks are available as command line
 java {
     withSourcesJar()
     withJavadocJar()
-    sourceCompatibility = 1.11
-    targetCompatibility = 1.11
+    sourceCompatibility = JavaVersion.VERSION_11
+    targetCompatibility = JavaVersion.VERSION_11
 }
 
 repositories {