update CI runner to ubuntu 20.04

Author: sjpb

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
     # See:
     #   - https://bugzilla.redhat.com/show_bug.cgi?id=190144
     #   - https://github.com/systemd/systemd/pull/16424
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix: