ci: update node/actions due to GH node v20 deprecation

Still on node20 - blocked on upstream releases:

peaceiris/actions-hugo@v3 - the maintainer merged a node24 upgrade to main but hasn't cut a new release tag yet; nothing to do until v3.1.0 drops
HugoBlox/gh-action-updater@v2 - needs update

templates/documentation/.github/workflows/build.yml

@@ -46,7 +46,7 @@ jobs:
 
     - name: Setup pnpm
       if: hashFiles('package.json') != ''
-      uses: pnpm/action-setup@v4
+      uses: pnpm/action-setup@v6
 
     - name: Get Hugo Version
       id: hugo-version