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>

tests/integration/api_httpsig_test.go

@@ -95,9 +95,7 @@ func TestHTTPSigCert(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()
 	session := loginUser(t, "user1")
 
-	csrf := GetUserCSRFToken(t, session)
 	req := NewRequestWithValues(t, "POST", "/user/settings/keys", map[string]string{
-		"_csrf":   csrf,
 		"content": "user1",
 		"title":   "principal",
 		"type":    "principal",