Change cursor to real cursor to avoid performance issue in Admin API

When fixing the issue https://github.com/authgear/authgear-server/issues/3176
It's been brought up that the cursor we are using in the admin API is an offset. This may cause performance issue.

Start by reading issue #3176 and locating the Admin API pagination implementation that currently represents a cursor as an offset. Trace how the cursor is generated and consumed, then verify that the completed change uses a real cursor and avoids the stated performance issue.

Written by the indexing model from the issue text.