Use the ubuntu-20.04 runner as 18.04 is deprecated

Author: cknitt

.github/workflows/ci.yml

@@ -11,7 +11,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [macos-latest, macos-arm, ubuntu-18.04, windows-latest]
+        # Stay on the oldest Ubuntu version that's still supported by Github Actions
+        # to avoid glibc incompatibilities as far as possible.
+        os: [macos-latest, macos-arm, ubuntu-20.04, windows-latest]
         # syntax explanation:
         # https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#example-including-additional-values-into-combinations
         include: