Update github workflow to v4 from v2 based on deprecation

Author: liamjberrisford

.github/workflows/deploy-book.yml

@@ -39,7 +39,7 @@ jobs:
 
     # Upload the book's HTML as an artifact
     - name: Upload artifact
-      uses: actions/upload-pages-artifact@v2
+      uses: actions/upload-pages-artifact@v4
       with:
         path: "_build/html"