action: remove the py38 support in ci

we use container fedora-python-tox for ci, which is not supporting py38
anymore, and ci is broken, like this:

py38-django3: skipped because could not find python interpreter with spec(s): py38
https://github.com/release-engineering/kobo/actions/runs/21279154145/job/61847476854?pr=278

Let's remove the py38 support in ci.

Author: dichn

.github/workflows/main.yml

@@ -27,15 +27,12 @@ jobs:
           # You can get it from `tox -l | sed "s/$/,/"`
           py36-django2,
           py36-django3,
-          py38-django2,
-          py38-django3,
           py39-django2,
           py39-django3,
           py310-django2,
           py310-django3,
           py311-django2,
           py311-django3,
-          py38-django4,
           py39-django4,
           py310-django4,
           py311-django4,