jupyter notebook: Unrecognized flag: '--existing'

Hello.

This is a copy of an [issue #4373 in jupyter notebook](https://github.com/jupyter/notebook/issues/4373). It was suggested that the problem might be not with the notebook, but with the jupyter kernel.

The magic `%connect_info` mentioned in the [manual](https://jupyter-notebook.readthedocs.io/en/stable/examples/Notebook/Connecting%20with%20the%20Qt%20Console.html) provides with the info of the kernel following with a few usage suggestions:
```
{
"shell_port": 53512,
"iopub_port": 53513,
"stdin_port": 53514,
"control_port": 53515,
"hb_port": 53516,
"ip": "127.0.0.1",
"key": "0a75d930-a42c67004b5ae820150b073f",
"transport": "tcp",
"signature_scheme": "hmac-sha256",
"kernel_name": ""
}

Paste the above JSON into a file, and connect with:
$> jupyter --existing
or, if you are local, you can connect with just:
$> jupyter --existing kernel-ac8f42fc-4851-4367-93cc-24251f01abbc.json
or even just:
$> jupyter --existing
if this is the most recent Jupyter kernel you have started.
```
Unfortunately, running `jupyter notebook --existing ...` raises an error
```
[C 21:59:25.219 NotebookApp] Bad config encountered during initialization:
[C 21:59:25.219 NotebookApp] Unrecognized flag: '--existing'
```

Versions:
```
jupyter-client 5.3.1
jupyter-contrib-core 0.3.3
jupyter-core 4.5.0
notebook 5.7.2
```

Best regards
Vladimir