Repo Gardening: update triage tasks (wording and Slack notifications)

[jeherve]

Proposed changes:

This PR introduces changes to 2 different triage tasks:

1. For the task posting on Slack when issues have gathered more than 10 tickets:
   • Notify the Kitkat team by default.
   • Update the Slack message format to match Update to the KitKat alerts to remove emojis, and simplify the markdown. #29636.
2. For the issue triage:
   • Notify Kitkat of issues that do not have a Priority label yet.
   • For such issues, and until Kitkat intervenes, add a "[Pri] TBD" label. This serves 2 purposes: it helps us detect issues that have triggered a Slack notification so we do not do that multiple times, and it may incite Automatticians to add the missing Priority label on their own.

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

• p1679680635165249-slack-CQD1HH4MA

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

• No

Testing instructions:

• This is hard to test before merging, so I will be testing in a separate repo.
  • Here is how the new message looks like: p1679912155246299-slack-CN2FSK7L4

[github-actions]

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[jeherve]

I may want to extract this function into a helper that can be shared by the different tasks, since I have a few variations of it already.

[anomiex]

Let's try it