Fix release URL in webhooks (#27182) (#27184)

Backport #27182 by @jolheiser

Resolves #27180

`URL` points to the API URL, `HTMLURL` points to the web page.

Notably, however, for PRs they are the same URL. I switched them to use
HTMLURL to match the rest of the codebase terminology.

Co-authored-by: John Olheiser <john.olheiser@gmail.com>

services/webhook/general_test.go

@@ -228,7 +228,7 @@ func pullReleaseTestPayload() *api.ReleasePayload {
 			Target:  "master",
 			Title:   "First stable release",
 			Note:    "Note of first stable release",
-			URL:     "http://localhost:3000/api/v1/repos/test/repo/releases/2",
+			HTMLURL: "http://localhost:3000/test/repo/releases/tag/v1.0",
 		},
 	}
 }