Allow KeepAlive controller Ping method to be requested by non local requests (#10126)

* Allow KeepAlive controller Ping method to be requested by non local requests and accept head requests

* removed unused references

src/Umbraco.Web/Editors/KeepAliveController.cs

@@ -1,14 +1,12 @@
 using System.Runtime.Serialization;
 using System.Web.Http;
-using Umbraco.Web.Mvc;
 using Umbraco.Web.WebApi;
-using Umbraco.Web.WebApi.Filters;
 
 namespace Umbraco.Web.Editors
 {
     public class KeepAliveController : UmbracoApiController
     {
-        [OnlyLocalRequests]
+        [HttpHead]
         [HttpGet]
         public KeepAlivePingResult Ping()
         {