Two things I missed while implementing [MSC2140](https://github.com/matrix-org/matrix-doc/pull/2140/files#diff-c03a26de5ac40fb532de19cb7fc2aaf7R80). 1. Access tokens should be provided to the identity server as `access_token`, not `id_access_token`, even though the homeserver may accept the tokens as `id_access_token`. 2. Access tokens must be sent to the identity server in a query parameter, the JSON body is not allowed. We now send the access token as part of an `Authorization: ...` header, which fixes both things. The breaking code was added in https://github.com/matrix-org/synapse/pull/5892 Sytest PR: https://github.com/matrix-org/sytest/pull/697
