GH Actions: various updates

[jrfnl]

GH Actions: fix use of deprecated set-output

GitHub has deprecated the use of set-output (and set-state) in favour of new environment files.

This commit updates workflows to use the new methodology.

Refs:

• https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
• https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#environment-files

GH Actions: update the xmllint-problem-matcher

The xmllint-problem-matcher action runner has released a new version which updates it to use node 16.
This gets rid of a warning which was shown in the action logs.

Note: I've suggested to the author to use long-running branches for the action runner instead, which would make this update redundant, but no telling if or when they'll respond to that, let alone if they will follow my suggestion.

Refs:

• https://github.com/korelstar/xmllint-problem-matcher/releases/tag/v1.1

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👉🏻 Note: this won't get rid of all warning yet as a lot of predefined action runners also use set-output, but most of those are in the process of updating and/or have released a new version already, so the other warnings should automatically disappear over the next few weeks.

[jrfnl]

I've removed the second commit - the action runner branch has become a long-running branch, so this update is no longer needed.

[GaryJones]

✅