Labels: update requirements for the "[Type]" labels

[jeherve]

Fixes ARC-141

Proposed changes:

With the move to Linear, we no longer use the "[Type]" prefix for our type labels.. Let's update our requirements accordingly.

Notes

• I will also do the same for other type labels in follow-up PRs.
• We will continue to use a "Bug" label, but without the prefix because in Linear there is a concept of parent <> child label.
• Once this is merged, I will update the labels in use in this repo and in other repos using the Repo Gardening action.
• I will also update other tools and scripts that may expect a prefix.

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

• pdXTV4-KR-p2

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

• no

Testing instructions:

Check for typos and other uses of the label.

[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:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Review, ...).
  
• ✅ Add a "[Type]" label (Bug, Enhancement, Janitorial, Task).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

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[tbradsha]

Seems reasonable, though I wonder if during the check we should also gracefully convert any [Type] Bug labels to Bug?