Update Python version matrix in CI workflow

Author: pgleeson

.github/workflows/omv-ci.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [ 3.8, 3.9 ]
+        python-version: [ 3.9, 3.12 ]
         engine: ["NEURON:8.1", "PyNEURON:8.1", "jNeuroML", "jNeuroML_NEURON", "jNeuroML_NetPyNE", "jNeuroML_validate"]