remove Python 3.8 support as soon EOL

[shalberd]

Is your feature request related to a problem? Please describe.
I recently updated nbconvert in a Pull Request due to problems with a library dependency in earlier versions.
See #3240

In the course of that, I also updated some core jupyter libraries to be more recent, in line with that Red Hat Open Data Hub Contrib Workbench Images is using in the context of Jupyterlab on Python 3.9 and 3.11.
Tests / Checks failed because certain new library versions require at least Python 3.9 in the python environment.

Describe the solution you'd like
Remove all support for Python 3.8 from tests and image environment validation.
Similar to what was done back then to remove Python 3.7 support https://github.com/elyra-ai/elyra/pull/3101/files#diff-7b84ea4a0a5058af570a2d621f94cf210464b0e8ce04c6d7bd24c614b38e0abb
Describe alternatives you've considered
Not as big of an issue for Jupyterlab 3.x, but probably all the more so for Jupyterlab 4.x support in future.
Additional context
I don't know anyone still using Python 3.8. Its end of life is in October 2024
https://devguide.python.org/versions/

After we have removed Python 3.8 support, we can make Python 3.11 work correctly #3199

[shalberd]

@lresende during merge, for Python 3.8 "validate image environment", the following error showed up on Aug 22

https://github.com/elyra-ai/elyra/actions/runs/10498724726/job/29084212287

that error did not show in my PR in the same step, seems like it only appears post-merge?

https://github.com/elyra-ai/elyra/actions/runs/10471757923/job/28999708860

conda env remove -y -n elyra-image-env
conda create -y -n elyra-image-env python=3.8 --channel conda-forge
Channels:

• conda-forge
• defaults
  Platform: linux-64
  Collecting package metadata (repodata.json): ...working... failed

UnavailableInvalidChannel: HTTP 403 FORBIDDEN for channel conda-forge https://conda.anaconda.org/conda-forge
The channel is not accessible or is invalid.

I don't think it's about conda and python 3.8, probably more a config issue.

https://stackoverflow.com/questions/66689887/unavailableinvalidchannel-the-channel-is-not-accessible-or-is-invalid-error-co

I don't see a pattern for the failed tests (red x). https://github.com/elyra-ai/elyra/commits/main/
Sometimes it is also cypress. Maybe test timing issues?

@harshad16 what do you think?

[lresende]

conda create -y -n elyra-image-env python=3.8 --channel conda-forge

should be python=3.9?

And there are some config options for configuring conda-forge channels in https://mamba.readthedocs.io/en/latest/user_guide/troubleshooting.html#defaults-channels