Remove usage of deprecated `set-output` command

[mhjort]

GitHub Actions is deprecating set-output behaviour. This task will stop working after 31st May 2023. Recommended way is to use new GITHUB_OUTPUT environment variable instead.

Here is a link to the announcement: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

[aboutte]

@ryanlitalien

[iakoff]

In a week there will be an anniversary.