Use the latest available Ubuntu image to run tests

It is the Ubuntu 20.04 version at the time of writing.

Github Actions has deprecated the 18.04 we were using.

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ permissions:
 
 jobs:
   tests:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     timeout-minutes: 60
     services:
       postgres:
@@ -65,7 +65,7 @@ jobs:
   coveralls:
     permissions:
       contents: none
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     needs: tests
     env:
       CI_BUILD_NUMBER: ${{ github.run_number }}