fix openAPI version parameter in gradle plugin

Author: frantuma

modules/swagger-gradle-plugin/src/main/java/io/swagger/v3/plugins/gradle/tasks/ResolveTask.java

@@ -131,7 +131,7 @@ public enum Format {JSON, YAML, JSONANDYAML}
 
     @Input
     @Optional
-    private String openAPIVersion;
+    private Property<String> openAPIVersion = getProject().getObjects().property(String.class);;
 
     @Input
     @Optional