fix: remove old saved openid provider settings from cache when starting Vikunja

Resolves https://kolaente.dev/vikunja/api/issues/1487

pkg/modules/auth/openid/providers.go

@@ -161,3 +161,7 @@ func getProviderFromMap(pi map[string]interface{}) (provider *Provider, err erro
 
 	return
 }
+
+func CleanupSavedOpenIDProviders() {
+	_ = keyvalue.Del("openid_providers")
+}