Use latest Ubuntu runner

Author: kytta

.github/workflows/test.yml

@@ -13,7 +13,7 @@ env:
 jobs:
   build:
     name: build (Python ${{ matrix.python-version }}, Django ${{ matrix.django-version }})
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     strategy:
       fail-fast: false
       matrix: