Migrate from CAS to Entra-ID

[tpendragon]

Summary or User Story

We've been asked to move our applications from CAS to Entra-ID by the end of the calendar year, barring any significant blockers.

Documentation for how to add an Entra-ID app. We would be adding an OIDC connection for this app.

Acceptance Criteria

• Login continues to work.
• OIDC accounts map to previously existing CAS accounts.

First step

Read the docs, find the relevant library - probably https://github.com/omniauth/omniauth_openid_connect?