Deprecate 'whitelist' and replace with 'allowed_users`

[ianhinder]

If I follow the example in the README, and set

c.CASLocalAuthenticator.whitelist = {'foo'}

I get a warning

jupyterhub-jupyterhub-1  | [W 2023-03-17 16:34:31.450 JupyterHub auth:175] CASLocalAuthenticator.whitelist is deprecated in JupyterHub 1.2, use CASLocalAuthenticator.allowed_users instead

However, if I replace whitelist with allowed_users, while the error message goes away, the server is now open to any user, since jhub_cas_authenticator is only checking the "whitelist" variable.

I propose that the check against whitelist is replace with a check against either variable.

[cwaldbieser]

I just pushed a change to master. Can you test if that works how you expect? It's a small change and I am pretty confident it should work. Even so, I'd like a test before I cut a release.