Fix error with test conventions on tasks that require Docker (#42719)

Author: mark-vieira

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -260,7 +260,7 @@ class BuildPlugin implements Plugin<Project> {
         if (ext.get('buildDocker')) {
             (ext.get('requiresDocker') as List<Task>).add(task)
         } else {
-            task.enabled = false
+            task.onlyIf { false }
         }
     }