Add JS maps to the GBM release

[jhnstn]

This would replace these steps from the release checklist

Navigate to the Buildkite job that built the JS bundles (Build JS Bundles) for the published tag. Open the job and navigate to the “Artifacts” tab. Locate the composed source maps (they have file name bundle/{platform}/App.composed.js.map) and download them.

Navigate and edit the GitHub release. Attach the composed source maps to the release (you can drag and drop the files in the “Attach binaries” drop area). Once they are uploaded, update the artifact’s name following this format:

[jhnstn]

The current build pipeline on GBM generates these maps and uploads them to Buildkite. (h/t @fluiddot )
We should be able to use gh release upload in that workflow (docs)