Add action to close stale PR

[rdwebdesign]

What does this PR aim to accomplish?

Add an action to verify every PR marked with Merge Conflict label and automatically close it.

How the action works

The action will close (actually, mark as stale and close immediately) any PR that have been labeled with 'merge conflict' 30 (or more) days earlier. This action will run once a day.

The action will ignore:

• every PR without Merge Conflict label;
• every PR with internal, never-stale, ON HOLD, WIP labels;
• draft PR.

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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)
6. I have checked that another pull request for this purpose does not exist.
7. I have considered, and confirmed that this submission will be valuable to others.
8. I accept that this submission may not be used, and the pull request closed at the will of the maintainer.
9. I give this submission freely, and claim no ownership to its content.

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