fix: deprecated versions of actions/upload-artifact

The pages workflow is failing due to deprecated versions of
actions/upload-artifact:
https://github.com/bazel-contrib/rules_foreign_cc/actions/runs/11706830028/job/32604817856

For more info, see:
 * https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/
 * https://github.blog/changelog/2024-02-13-deprecation-notice-v1-and-v2-of-the-artifact-actions/
 * https://github.com/actions/upload-artifact

.github/workflows/pages.yaml

@@ -67,7 +67,7 @@ jobs:
         run: ${{ github.workspace }}/bin/mdbook build
         working-directory: ${{ github.workspace }}/docs
       - name: Save the newly built book
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v4
         with:
           name: "${{ matrix.ref }}"
           path: ${{ github.workspace }}/docs/book
@@ -78,7 +78,7 @@ jobs:
         if: ${{ matrix.ref == 'main' }}
         working-directory: ${{ github.workspace }}/docs/root
       - name: Save the newly built book
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v4
         with:
           name: "${{ matrix.ref }}-root"
           path: ${{ github.workspace }}/docs/root/book