Do not try to remove stale label on PRs

[yubiuser]

PR master branch!

What does this PR aim to accomplish?:

PR #1552 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.
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5. I have squashed any insignificant commits. (git rebase)
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• I have read the above and my PR is ready for review. Check this box to confirm

[yubiuser]

Second commit should fix

[DL6ER]

@PromoFaux @dschaper Sorry for the ping, but this needs to go to master, hence a third pair of eyes needs to say: Yes.