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>

routers/web/auth/oauth.go

@@ -393,9 +393,6 @@ func handleOAuth2SignIn(ctx *context.Context, authSource *auth.Source, u *user_m
 			return
 		}
 
-		// force to generate a new CSRF token
-		ctx.Csrf.PrepareForSessionUser(ctx)
-
 		if err := resetLocale(ctx, u); err != nil {
 			ctx.ServerError("resetLocale", err)
 			return