Update actions/download-artifact action from v4 to v5 (major v5)

Author: renovate[bot]

.github/workflows/ci.yml

@@ -239,7 +239,7 @@ jobs:
     steps:
 
       - name: "Download base reference."
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v5
         with:
           name: 'github-pages-base'
           path: github-pages-base
@@ -255,7 +255,7 @@ jobs:
           sed -re 's/Last Updated: [0-9]{4}\/[0-9]{2}\/[0-9]{2}//' -i humans.txt
 
       - name: "Download just-built artifact."
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v5
         with:
           name: 'github-pages'
           path: github-pages-head