Use GITHUB_OUTPUT envvar instead of set-output command as the latter is deprecated

[arunsathiya]

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

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[richardm-stripe]

Looks good to me! Thanks for including the documentation links so this change was easy to understand and verify.