Update HTTP status codes to modern codes (#18063)

* 2xx/3xx/4xx/5xx -> http.Status...
* http.StatusFound -> http.StatusTemporaryRedirect
* http.StatusMovedPermanently -> http.StatusPermanentRedirect

routers/web/repo/issue_watch.go

@@ -53,5 +53,5 @@ func IssueWatch(ctx *context.Context) {
 		return
 	}
 
-	ctx.Redirect(issue.HTMLURL(), http.StatusSeeOther)
+	ctx.Redirect(issue.HTMLURL())
 }