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/auth/signup_inner.tmpl

@@ -11,7 +11,6 @@
 			<p>{{ctx.Locale.Tr "auth.sign_up_tip"}}</p>
 		{{end}}
 		<form class="ui form" action="{{.SignUpLink}}" method="post">
-			{{.CsrfTokenHtml}}
 			{{if or (not .LinkAccountMode) (and .LinkAccountMode .LinkAccountModeRegister)}}
 			{{template "base/alert" .}}
 			{{end}}