Reduce the size of the gh-pages branch

https://github.com/peaceiris/actions-gh-pages?tab=readme-ov-file#%EF%B8%8F-force-orphan-force_orphan

> We can set the force_orphan: true option. This allows you to make your
> publish branch with only the latest commit.

On its own, this won't remove the gh-pages history, but once the branch
is pointing at an orphaned commit we should be able to remove the
references to the old gh-pages commits and garbage collect them.

.github/workflows/deploy.yml

@@ -49,6 +49,7 @@ jobs:
           publish_dir: ./build
           cname: docs.publishing.service.gov.uk
           commit_message: ${{steps.commit_message_writer.outputs.commit_message}}
+          force_orphan: true
       - name: Notify failure
         uses: slackapi/slack-github-action@v1
         if: ${{ failure() }}