Bump GH runner before ubuntu-18.04 is removed

Author: hugovk

.github/workflows/test.yml

@@ -6,7 +6,7 @@ on: [push, pull_request]
 jobs:
   build:
     name: build (Python ${{ matrix.python-version }}, Django ${{ matrix.django-version }})
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
 
     strategy:
       fail-fast: false