Fix deprecated set-output command

[kjaymiller]

originally submitted by @arunsathiya - Azure-Samples/azure-flask-postgres-flexible-appservice#24

save-state and set-output commands used in GitHub Actions are deprecated and GitHub recommends using environment files.

This PR updates the usage of ::set-output to "$GITHUB_OUTPUT"

Instructions for envvar usage from GitHub docs:

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-output-parameter

[kjaymiller]

@arunsathiya - would you like to make this change or let one of the maintainers fix it?