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/oauth2.go

@@ -133,7 +133,7 @@ func loadOAuth2From(rootCfg ConfigProvider) {
 
 	// FIXME: at the moment, no matter oauth2 is enabled or not, it must generate a "oauth2 JWT_SECRET"
 	// Because this secret is also used as GeneralTokenSigningSecret (as a quick not-that-breaking fix for some legacy problems).
-	// Including: CSRF token, account validation token, etc ...
+	// Including: account validation token, etc ...
 	// In main branch, the signing token should be refactored (eg: one unique for LFS/OAuth2/etc ...)
 	jwtSecretBase64 := loadSecret(sec, "JWT_SECRET_URI", "JWT_SECRET")
 	if InstallLock {