ci: Use GITHUB_OUTPUT envvar instead of set-output command

[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

[UlisesGascon]

@inigomarquinez can u have a look?

[inigomarquinez]

Approved! In fact, in other PRs we already use the approach suggested by @arunsathiya . Thanks for the catch!