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/pull_comment_test.go

@@ -89,7 +89,6 @@ func testPullCommentRetarget(t *testing.T, u *url.URL, session *TestSession) {
 
 	// do retarget
 	req := NewRequestWithValues(t, "POST", fmt.Sprintf("/user2/repo1/pull/%d/target_branch", prIssue.PullRequest.Index), map[string]string{
-		"_csrf":         GetUserCSRFToken(t, session),
 		"target_branch": "test-branch/retarget-no-conflict",
 	})
 	session.MakeRequest(t, req, http.StatusOK)