CI: Temporarily force ubuntu-24.04

In order to get the latest pandoc. GitHub is rolling out 24.04 as the
default for "ubuntu-latest" between Dec 05 and next January.

.github/workflows/publish-web.yml

@@ -9,7 +9,7 @@ on: [push, pull_request, workflow_dispatch]
 jobs:
   publish:
     name: Publish
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04
     steps:
     - uses: actions/checkout@v4