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/slack.go

@@ -223,7 +223,7 @@ func (s *SlackPayload) PullRequest(p *api.PullRequestPayload) (api.Payloader, er
 		attachments = append(attachments, SlackAttachment{
 			Color:     fmt.Sprintf("%x", color),
 			Title:     issueTitle,
-			TitleLink: p.PullRequest.URL,
+			TitleLink: p.PullRequest.HTMLURL,
 			Text:      attachmentText,
 		})
 	}