'port' moved from NotebookApp to ServerApp, but Jupyter ignores ServerApp.port

[fofoni]

Description
Today I installed Jupyter in a new virtual environment, and when started, it displayed the following warning:

[W 2021-02-05 15:12:00.626 LabApp] 'port' has moved from NotebookApp to ServerApp. This config will be passed to ServerApp. Be sure to update your config before our next release.

It still worked as usual. After I changed the line:

c.NotebookApp.port = <my custom port>

in ~/.jupyter/jupyter_notebook_config.py to:

c.ServerApp.port = <my custom port>

the warning is no more, but now Jupyter ignores the configuration and listens at the default 8888 port.

The same problem happened to other configuration variables, like password.

To Reproduce

1. Change the setting NotebookApp.port to ServerApp.port in the file ~/.jupyter/jupyter_notebook_config.py
2. Run jupyter notebook, and check that it outputs [I 20:16:38.282 NotebookApp] http://localhost:8888/?token=<...>

Expected behavior
The warning message 'port' has moved from NotebookApp to ServerApp. This config will be passed to ServerApp. Be sure to update your config led me to believe that, if I changed the config from NotebookApp to ServerApp, everything would still work as usual (in particular, Jupyter would listen at my custom port as specified in ServerApp.port).

Version info:
Fedora 31 Linux 5.5.13-200.fc31.x86_64 #1 SMP Wed Mar 25 21:55:30 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

$ jupyter --version
jupyter core     : 4.7.1
jupyter-notebook : 6.2.0
qtconsole        : 5.0.2
ipython          : 7.20.0
ipykernel        : 5.4.3
jupyter client   : 6.1.11
jupyter lab      : 3.0.7
nbconvert        : 6.0.7
ipywidgets       : 7.6.3
nbformat         : 5.1.2
traitlets        : 5.0.5

$ python
Python 3.9.1 (default, Feb  5 2021, 15:04:29) 
[GCC 9.3.1 20200408 (Red Hat 9.3.1-2)] on linux

[Zsailer]

@meeseeksdev migrate to jupyter-server/jupyter_server

[lumberbot-app]

Done as jupyter-server/jupyter_server#434.