Replace unmaintained upload-release-asset action

[ibLeDy]

The action we are currently using to upload assets to a release is not being actively maintained, so we need to replace it.

We recently replaced https://github.com/actions/create-release with https://github.com/softprops/action-gh-release, which already has the option to upload assets (it would also allow us to do it in a single step).

[ibLeDy]

The action just failed when pushing a new tag.

https://github.com/ibLeDy/timezone-converter/runs/2560560948?check_suite_focus=true