Replace CSRF cookie with CrossOriginProtection (#36183)

Removes the CSRF cookie in favor of
[`CrossOriginProtection`](https://pkg.go.dev/net/http#CrossOriginProtection)
which relies purely on HTTP headers.

Fixes: https://github.com/go-gitea/gitea/issues/11188
Fixes: https://github.com/go-gitea/gitea/issues/30333
Helps: https://github.com/go-gitea/gitea/issues/35107

TODOs:

- [x] Fix tests
- [ ] Ideally add tests to validates the protection

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/settings/webhook/slack.tmpl

@@ -1,7 +1,6 @@
 {{if eq .HookType "slack"}}
 	<p>{{ctx.Locale.Tr "repo.settings.add_web_hook_desc" "https://slack.com" (ctx.Locale.Tr "repo.settings.web_hook_name_slack")}}</p>
 	<form class="ui form" action="{{.BaseLink}}/slack/{{or .Webhook.ID "new"}}" method="post">
-		{{.CsrfTokenHtml}}
 		<div class="required field {{if .Err_PayloadURL}}error{{end}}">
 			<label for="payload_url">{{ctx.Locale.Tr "repo.settings.payload_url"}}</label>
 			<input id="payload_url" name="payload_url" type="url" value="{{.Webhook.URL}}" autofocus required>