Repo gardening: don't send slack notifs for closed issues

[jeherve]

Proposed changes:

This should avoid slack notifications like this one:

p1737134904935319/1737134740.218539-slack-C07418EJ0

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

• N/A

Does this pull request change what data or activity we track or use?

No

Testing instructions:

This can only be tested in a fork I'm afraid. I cannot link to a Slack notification since this stops sending notifs. :)

[github-actions]

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