Update to lookup default display name has reduced use of first name in sync

As a result of https://gitlab.developers.cam.ac.uk/uis/devops/grails-application-ansible-deployment/-/merge_requests/197, the default display name for ~1k people changed to pref initial + surname.

The naming code in this sync assumes that a set display name (not equal to registered name) is to be used in preference to using actual pref first name.

Pipeline with reduction: https://gitlab.developers.cam.ac.uk/uis/devops/gsuite/sync-deploy/-/jobs/700590

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/ibis/ibis/-/issues/123