CI Test: Deprecating 'set-output' command

[rdrll]

Description

This PR replaced the deprecating set-output command used in the diff_shades_comment GitHub Action with latest GitHub supported method.

See this GitHub blog about this deprecation.

The original method of setting variables directly using the set-output command has now been replaced with setting variables through the GITHUB_OUTPUT environment variable.

After this fix, there will be no more warnings like this popping up when executing this CI test:

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary? not necessary
• Add new / update outdated documentation? not necessary

[JelleZijlstra]

Thanks!