Update quality checklist to include UX label

What does this MR do and why?

Update quality checklist (https://docs.gitlab.com/ee/development/code_review.html#acceptance-checklist) to include UX label

We still have too many cases where designers are not getting pinged on MR reviews, and calling out the UX label in that process might help us already right now, while also setting us up for better tracking and automation.

Closes #343470 (closed)

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.