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/migrate_test.go

@@ -95,7 +95,6 @@ func TestMigrateGiteaForm(t *testing.T) {
 		// Step 4: submit the migration to only migrate issues
 		migratedRepoName := "otherrepo"
 		req = NewRequestWithValues(t, "POST", link, map[string]string{
-			"_csrf":       htmlDoc.GetCSRF(),
 			"service":     fmt.Sprintf("%d", structs.GiteaService),
 			"clone_addr":  fmt.Sprintf("%s%s/%s", u, ownerName, repoName),
 			"auth_token":  token,