Use ubuntu-latest in Docker publish step

Seems 20.04 has been removed.

Author: code-asher

.github/workflows/publish.yaml

@@ -145,7 +145,7 @@ jobs:
           gh pr create --repo coder/code-server-aur --title "chore: bump version to ${{ env.VERSION }}" --body "PR opened by @$GITHUB_ACTOR" --assignee $GITHUB_ACTOR
 
   docker:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout code-server
         uses: actions/checkout@v4