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/user/settings/keys_principal.tmpl

@@ -42,7 +42,6 @@
 		</h4>
 		<div class="ui attached segment">
 			<form class="ui form" action="{{.Link}}" method="post">
-				{{.CsrfTokenHtml}}
 				<div class="field {{if .Err_Content}}error{{end}}">
 					<label for="ssh-principal-content">{{ctx.Locale.Tr "settings.principal_content"}}</label>
 					<input id="ssh-principal-content" name="content" value="{{.content}}" autofocus required>