fix: Backchannel Logout: Fix error log when RP responds with status c…

…ode 204

According to the specification at https://openid.net/specs/openid-connect-backchannel-1_0.html#BCResponse
the Relying Party must respond with a status code 200.
However, it also notes that the OpenID Provider should be prepared to
handle status code 204 (No Content) as a successful response as well.

consent/strategy_default.go

@@ -791,8 +791,8 @@ func (s *DefaultStrategy) executeBackChannelLogout(r *http.Request, subject, sid
 		}
 		defer res.Body.Close()
 
-		if res.StatusCode != http.StatusOK {
-			log.WithError(errors.Errorf("expected HTTP status code %d but got %d", http.StatusOK, res.StatusCode)).
+		if res.StatusCode != http.StatusOK && res.StatusCode != http.StatusNoContent {
+			log.WithError(errors.Errorf("expected HTTP status code %d or %d but got %d", http.StatusOK, http.StatusNoContent, res.StatusCode)).
 				Error("Unable to execute OpenID Connect Back-Channel Logout Request")
 			return
 		} else {