/users/:user_id/admin endpoint is disabled on a HS with MSC3861 configured

[teutat3s]

isSynapseAdmin() currently does not work on a HS with OIDC setup according to MSC3861 (e.g. using matrix-authentication-service).

draupnir logs:

[ERROR] [ActionException] ActionException Unknown <uuid> Unable to query whether the user @draupnir:<domain> is a Synapse Admin Error: Error during MatrixClient request GET /_synapse/admin/v1/users/%40draupnir%3A<domain>/admin: 404 Not Found -- {"errcode":"M_UNRECOGNIZED","error":"Unrecognized request"}

GET /_synapse/admin/v1/users/<user_id>/admin
Note: This API is disabled when MSC3861 is enabled. See #15582

Source: https://element-hq.github.io/synapse/latest/admin_api/user_admin_api.html#get-whether-a-user-is-a-server-administrator-or-not

See also: matrix-org/synapse#15582