Update GH action upload artifact to v4

Author: PhilReedData

.github/workflows/main.yml

@@ -31,7 +31,7 @@ jobs:
         run: jupyter-book build --builder dirhtml TeSS_docs/
 
       - name: Upload artifact
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           path: "_build/dirhtml"