Team assignment: update GitHub issue mapping for Newsletter to Loop team

[holdercp]

Proposed changes:

• Updates the GitHub issue mapping between teams and labels so that Newsletter maps to the Loop team

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

p1738665051284949-slack-C083ZPVVDTK

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

Testing instructions:

I don't think we can't really test this until it's merged so just double-check the values are correct. Refer to the Slack discussion.

• You can test the Slack channel id is correct by using the web Slack client and changing the URL.
• Visit the GitHub board to verify it's correct.

Once merged, we can validate things are working correctly by using these steps:

• Create a test GH issue for the Newsletter feature
  • Add a "[Block] Subscriber Login" or a "[Feature] Subscriptions" label
  • Make it high priority or blocker
• The test issue should appear on the Loop board -> Newsletter tab
• The creator newsletter slack should get a ping

[github-actions]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

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• ✅ Add a "[Status]" label (In Progress, Needs Team Review, ...).
  
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• ✅ Add changelog entries to affected projects
  

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