Upgrade singleuser.py to JupyterHub 4

[mahendrapaipuri]

jupyterhub-singleuser is a subclass of ExtensionApp and after a brief discussion with @minrk at JupyterCon, he told me that the new implementation of jupyterhub-singleuser does not expect any CLI args. If we use batchspawner with JupyterHub 4, we end up getting an error as follows:

[W 2023-05-13 15:27:12.537 ServerApp] Permission to listen on port 443 denied.
[C 2023-05-13 15:27:12.537 ServerApp] ERROR: the Jupyter server could not be started because port 443 is not available.
[D 2023-05-13 15:27:12.537 ServerApp] Exiting application: jupyter-server

The JUPYTERHUB_SERVICE_URL env var is set to http(s)://0.0.0.0:0/user/<user>/servers/<server> at the spawn time in batchspawner and due to this code here, depending on the HTTP scheme, the server will be either launched on 80/443 ports.

This PR replaces the port in JUPYTERHUB_SERVICE_URL with free port that we find and updates the env var. Consequently, CLI arg --port is removed as it is not used anymore.

[consideRatio]

@minrk are you able to help review this?

Note that batchspawner is in main branch currently requires jupyterhub 1.5.1+, up from the latest release that required jupyterhub 0.9+.

[minrk]

Updated the handling a tiny bit. JUPYTERHUB_SERVICE_URL is added in JupyterHub 2.0, so it's only 1.5.1 that needs the --port

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