updated test gradle versions

Author: hauner

src/testInt/groovy/com/github/hauner/openapi/gradle/OpenApiProcessorPluginSpec.groovy

@@ -116,7 +116,7 @@ openapiProcessor {
             '6.5', '6.5.1',
             '6.6', '6.6.1',
             '6.7', '6.7.1',
-            '6.8', '6.8.1', '6.8.2'
+            '6.8', '6.8.1', '6.8.2', '6.8.3'
         ].reverse ()
     }