chore(deps): update github artifact actions

Author: renovate[bot]

.github/workflows/build-antora.yml

@@ -31,7 +31,7 @@ jobs:
         run: npm run build
 
       - name: Upload built site
-        uses: actions/upload-artifact@v6
+        uses: actions/upload-artifact@v7
         with:
           name: antora-site
           path: build/site

.github/workflows/publish.yml

@@ -27,7 +27,7 @@ jobs:
 
     steps:
       - name: Download built site
-        uses: actions/download-artifact@v7
+        uses: actions/download-artifact@v8
         with:
           name: antora-site
           path: build/site