Add stale-issue-reason

[yubiuser]

• What does this PR aim to accomplish?:

Github recently added the issue-closed-reason feature (https://github.blog/changelog/2022-05-19-the-new-github-issues-may-19th-update/), which can be complete and not-planned. The latter is meant, among others, for stale.
This PR actions/stale#764 added the option close-issue-stale for the GH action stale to allow setting the issue-closed-reason.

This PR adds close-issue-stale: not_planned to our action.

Target: master branch

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

---

• I have read the above and my PR is ready for review. Check this box to confirm