Use of outdated `/_matrix/identity/api/v1/lookup` API

[richvdh]

The /_matrix/identity/api/v1 endpoints were removed from the matrix spec a while ago (see matrix-org/matrix-spec-proposals#3170) and we plan to (finally) drop it from Sydent (matrix-org/sydent#338).

In particular, people should not use the identity service without opting into its terms and conditions, which the v1 API does not support.