Update OS version used in publish step, as 18.04 is deprecated

Author: pimterry

.github/workflows/ci.yml

@@ -50,7 +50,7 @@ jobs:
           if-no-files-found: error
   publish:
     name: Publish a release
-    runs-on: "ubuntu-18.04"
+    runs-on: "ubuntu-20.04"
     if: github.event_name == 'push' && startsWith(github.ref, 'refs/tags/v')
     needs: build
     steps: