Bump GH runner to ubuntu-22.04

Python 3.5 and 3.6 are no longer supported in the test matrix as a
result.

Author: maxiberta

.github/workflows/tox.yml

@@ -21,10 +21,10 @@ jobs:
           POSTGRES_PASSWORD: django_app
         ports:
           - 5432:5432
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
-        python: [3.5, 3.6, 3.8, "3.10", "3.12"]
+        python: [3.8, "3.10", "3.12"]
 
     steps:
       - uses: actions/checkout@v2