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/issue/view_content/pull_merge_box.tmpl

@@ -384,7 +384,6 @@
 			{{if and $.StillCanManualMerge (not $showGeneralMergeForm)}}
 				<div class="divider"></div>
 				<form class="ui form form-fetch-action" action="{{.Issue.Link}}/merge" method="post">{{/* another similar form is in PullRequestMergeForm.vue*/}}
-					{{.CsrfTokenHtml}}
 					<div class="field">
 						<input type="text" name="merge_commit_id" placeholder="{{ctx.Locale.Tr "repo.pulls.merge_commit_id"}}">
 					</div>