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>

modules/setting/markup.go

@@ -255,7 +255,7 @@ func newMarkupRenderer(name string, sec ConfigSection) {
 	}
 
 	// ATTENTION! at the moment, only a safe set like "allow-scripts" are allowed for sandbox mode.
-	// "allow-same-origin" should never be used, it leads to XSS attack, and it makes the JS in iframe can access parent window's config and CSRF token
+	// "allow-same-origin" should NEVER be used, it leads to XSS attack: makes the JS in iframe can access parent window's config and send requests with user's credentials.
 	renderContentSandbox := sec.Key("RENDER_CONTENT_SANDBOX").MustString("allow-scripts allow-popups")
 	if renderContentSandbox == "disabled" {
 		renderContentSandbox = ""