Address GitHub deprecation to the method used to output

[MelSumner]

GitHub have deprecated the method used to output, that's probably the first port of call for deprecation fixes.

This might be related to the deprecations in the GitHub Octokit API.

[benschwarz]

We can use either actions core (core.setOutput) or set environment variable GITHUB_OUTPUT (see doc). Using actions core is prooooobably the way to go, but it's also potentially a butt load more effort to implement.

Maybe we start by setting GITHUB_OUTPUT for now?

[benschwarz]

Closed by #337