Upgrade pathway to Z2JH 4

[GeorgianaElena]

Z2JH 4.0.0 is out ↪ changelog and we should upgrade to using it.

Upgrade plan

1. Add support for Bootstrap 5 in jupyterhub-fancy-profiles
2. Open a draft PR with the infrastructure changes needed
3. Manually test a few representative deployments
4. Merge PR and rollout the new version to all hubs

Testing

Manually test:

1. Authentication
2. Server spawn
3. Home directory names are preserved
4. The configurator still works
5. New notebooks work
6. RStudio works
7. Dask works

After manually deploying the changes in the draft PR on the following hubs:

• [ ] 2i2c staging hub
• [ ] a persistent binderhub
• [ ] an ephemeral binderhub
• [ ] a hub that uses CILogon with GitHub
• [ ] a hub that uses CILogon with Google
• [ ] a hub that uses GoogleOAuthenticator directly
• [ ] jupyter-health staging
• [ ] a hub that uses an user image with JupyterHub 3
• [ ] a hub that uses an user image with JupyterHub 4
• [ ] a hub that uses an user image with JupyterHub 5

Definition of done

The jupyterhub-fancy-profiles uses botostrap 5, the changes in the draft PR have been tested per the strategy above, then the PR merged and all hubs use Z2JH 4 while the CI is happy.

[GeorgianaElena]

I intend to create sub-issues for each of them and further refine them and have this issue track the entire effort.