Do not try to remove stale label on PRs

[yubiuser]

PR Master branch

What does this PR aim to accomplish?:

PR #5252 added the workflow to remove the 'stale' label from issues immediately when a stale issue received new comments. However, the github event issue_comment also triggers on PR comments (see https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#issue_comment).

As we don't handle stale PR with this workflow PR write permission is not granted and the workflow will fail (see https://github.com/pi-hole/pi-hole/actions/runs/5109474838/jobs/9184306155)

This PR adds an additional check to verify the action is not triggered by comments on pull requests. (see https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#issue_comment-on-issues-only-or-pull-requests-only)

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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.
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9. I give this submission freely, and claim no ownership to its content.

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[yubiuser]

Second commit should fix

[PromoFaux]

This was merged into other repos, not sure how I missed this one