Replace set-output by GITHUB_OUTPUT

[felixfontein]

Fixes #56.

[briantist]

looks great so far, but looking at the annotations, may have missed some:

• https://github.com/ansible-community/github-docs-build/actions/runs/3254858651
• https://github.com/ansible-community/github-docs-build/actions/runs/3254858650

[felixfontein]

There's none more in this repository (I've grepped for it). Maybe some of these tests use actions from the main branch?

[briantist]

There's none more in this repository (I've grepped for it). Maybe some of these tests use actions from the main branch?

Ok so it turns out these are from other actions we're using. We should update setup-python to use v4: the breaking change in that version is that it no longer has a default python version so if we're not already, we'll need to specify one (no big deal). Would you mind updating that in this PR?

The other thing causing the warnings is download-artifact, which unfortunately doesn't have a fix yet, so we'll just have to wait for it and update it later if it ends up being a new major version:

• [bug] Warning in workflow about set-output actions/download-artifact#185

[briantist]

thanks @felixfontein !