GHA: don't test Plone 6.0 on Python 3.8.

Latest 6.0.14 no longer officially supports Python 3.8.
It really does not work anymore either, unless we would add an extra version pin:
Pillow 11 is pinned, and this does not work on Python 3.8.
See GHA failure here: https://github.com/collective/collective.exportimport/actions/runs/12652930616/job/35256797334?pr=249

.github/workflows/tests.yml

@@ -16,7 +16,6 @@ jobs:
         - ["3.6",   "plone52-py36"]
         - ["3.7",   "plone52-py37"]
         - ["3.8",   "plone52-py38"]
-        - ["3.8",   "plone60-py38"]
         - ["3.9",   "plone60-py39"]
         - ["3.10",  "plone60-py310"]
         - ["3.11",  "plone60-py311"]