Remove Python 3.8 from the test matrix for Plone 6.0, where it is no longer supported

Author: davisagli

.github/workflows/tests.yml

@@ -10,8 +10,6 @@ jobs:
         include:
           - python-version: "3.8"
             plone-version: "5.2"
-          - python-version: "3.8"
-            plone-version: "6.0"
           - python-version: "3.9"
             plone-version: "6.0"
           - python-version: "3.10"