Update github workflow to use upload/download artifact v4

v2 is giving an error: Error: This request has been automatically failed because it uses a deprecated version of . Learn more: https://github.blog/changelog/2024-02-13-deprecation-notice-v1-and-v2-of-the-artifact-actions/

Author: jasongrout

.github/workflows/validate.yml

@@ -34,7 +34,7 @@ jobs:
     # run: cp -r assets _site/assets
 
     - name: Publish built site
-      uses: actions/upload-artifact@v2
+      uses: actions/upload-artifact@v4
       with:
         name: Built site ${{ github.run_number }}
         path: ./_site
@@ -48,7 +48,7 @@ jobs:
 
     steps:
     - name: Fetch built site
-      uses: actions/download-artifact@v2
+      uses: actions/download-artifact@v4
       with:
         name: Built site ${{ github.run_number }}
         path: ./_site
@@ -67,7 +67,7 @@ jobs:
 
     steps:
     - name: Fetch built site
-      uses: actions/download-artifact@v2
+      uses: actions/download-artifact@v4
       with:
         name: Built site ${{ github.run_number }}
         path: ./_site
@@ -99,7 +99,7 @@ jobs:
       - name: Fetch repository for `lighthouserc.json`
         uses: actions/checkout@v2
       - name: Fetch built site
-        uses: actions/download-artifact@v2
+        uses: actions/download-artifact@v4
         with:
           name: Built site ${{ github.run_number }}
           path: ./_site