[EPIC] (DRAFT) Upgrade z2jh to v4 with jupyterhub v5, kubespawner v7, and oauthenticator v17

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This EPIC tracks upgrading z2jh 4. Currently 4.0.0-beta.1 is released, and work to get the final 4.0.0 release out is tracked upstream via jupyterhub/zero-to-jupyterhub-k8s#3406.

• JupyterHub 5 upgrade verification
  • Trial upgrade a staging hub and verify that currently running user servers keep running OK
  • Trial using an image with jupyterhub 3 installed
  • Trial using an image with jupyterhub 4 installed
• Kubespawner 7 verification
  • Trial upgrade a staging hub and verify home directories are retained by using subPath: "{escaped_username}" instead of subPath: "{username}"
• OAuthenticator 17 verification
  • Trial upgrade a staging hub involving github auth
  • Trial upgrade a staging hub involving google auth via cilogon