Do not test Plone 6.0 on Python 3.8. Do start testing 6.1.

Python 3.8 is not supported since Plone 6.0.14.

And it is too hard to use 6.0.13 on 3.8 and 6.0.14 on 3.9+: we would need a different requirements file for 3.8, otherwise pip fails with:

```
No matching distribution found for setuptools==75.6.0
```

.github/workflows/tests.yml

@@ -6,17 +6,25 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.12", "3.11", "3.10", "3.9", "3.8"]
-        plone-version: ["6.0", "5.2"]
+        python-version: ["3.13", "3.12", "3.11", "3.10", "3.9", "3.8"]
+        plone-version: ["6.1", "6.0", "5.2"]
         exclude:
+          - python-version: 3.13
+            plone-version: 5.2
           - python-version: 3.12
             plone-version: 5.2
           - python-version: 3.11
             plone-version: 5.2
           - python-version: 3.10
             plone-version: 5.2
+          - python-version: 3.9
+            plone-version: 6.1
           - python-version: 3.9
             plone-version: 5.2
+          - python-version: 3.8
+            plone-version: 6.1
+          - python-version: 3.8
+            plone-version: 6.0
     steps:
       # git checkout
       - uses: actions/checkout@v2

plone-6.0.x.cfg

@@ -2,7 +2,3 @@
 extends =
     https://dist.plone.org/release/6.0.14/versions.cfg
     base.cfg
-
-[buildout:python38]
-extends =
-    https://dist.plone.org/release/6.0.13/versions.cfg

plone-6.1.x.cfg

@@ -0,0 +1,4 @@
+[buildout]
+extends =
+    https://dist.plone.org/release/6.1.0b1/versions.cfg
+    base.cfg

requirements-6.1.txt

@@ -0,0 +1 @@
+-r https://dist.plone.org/release/6.1.0b1/requirements.txt