ci(github): change Python runners to range 3.10 to 3.12

This change drops Python versions 3.8 and 3.9, and adds version 3.12 to
the support matrix, in preparation for the switch to Django 5.0.

Author: ericswpark

.github/workflows/server-tests.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: ['3.8', '3.9', '3.10', '3.11']
+        python-version: ['3.10', '3.11', '3.12']
 
     env:
       SHIPPER_SECRET_KEY: "duk4hvedu3zxnmw@shnjw7z^)x#wcm@cj05t5f&#90rx-g44d2"