ci: upgrade upload-pages-artifact@v3 -> @v4

Author: hendriknielaender

.github/workflows/deploy-docs.yml

@@ -16,7 +16,7 @@ jobs:
       - run: zig build docs
 
       - name: Upload Pages artifact
-        uses: actions/upload-pages-artifact@v3
+        uses: actions/upload-pages-artifact@v4
         with:
           path: ./zig-out/docs