Bump upload-artifact version

Jobs seem to be randomly failing now that it’s deprecated; e.g. [1] gave

> Error: This request has been automatically failed because it uses a
> deprecated version of `actions/upload-artifact: v3`. Learn more:
> https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/

[1] https://github.com/ably/ably-cocoa/actions/runs/12694870211/job/35385596818?pr=2010

.github/workflows/check-pod.yaml

@@ -52,7 +52,7 @@ jobs:
           mkdir -p carthage-built-framework-artifact-contents/carthage-built-framework
           mv Ably.framework.zip carthage-built-framework-artifact-contents/carthage-built-framework
       - name: Archive built framework
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: carthage-built-framework
           path: carthage-built-framework-artifact-contents