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/html_helper.go

@@ -37,11 +37,6 @@ func (doc *HTMLDoc) Find(selector string) *goquery.Selection {
 	return doc.doc.Find(selector)
 }
 
-// GetCSRF for getting CSRF token value from input
-func (doc *HTMLDoc) GetCSRF() string {
-	return doc.GetInputValueByName("_csrf")
-}
-
 // AssertHTMLElement check if the element by selector exists or does not exist depending on checkExists
 func AssertHTMLElement[T int | bool](t testing.TB, doc *HTMLDoc, selector string, checkExists T) {
 	sel := doc.doc.Find(selector)