Label enforcer: add Gutenberg plugin to the list of single required labels

[priethor]

What?

Allows merging PRs that only have Gutenberg plugin label.

Why?

There are meta PRs that shouldn't need any extra label, like #65252

How?

By adding the label to the list of required labels in the label enforcer auatomation

Testing Instructions

1. Open a non-draft PR
2. Add the Gutenberg plugin label, and check that the PR can now passes the required label checks

[github-actions]

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Co-authored-by: priethor <priethor@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: vcanales <vcanales@git.wordpress.org>

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[vcanales]

Another option might be to enforce labels only on PRs that target trunk—#65252 targets a Release branch.

[vcanales]

Another option might be to enforce labels only on PRs that target trunk—#65252 targets a Release branch.

I'll go ahead and merge this, we can iterate over my suggestion later if necessary.