GlLabel: Remove size property from codebase

What does this MR do and why?

Integration branch of feat(label): Reduce label variants (gitlab-ui!3306 - merged)

• Removes custom label overrides tied to the :simplified_labels FF
• Removes :simplified_labels FF
• Removes size attribute/props from GlLabel's
• [TEMP] Adds latest GitLab UI package for testing until it's merged (!149775 (merged)) ✔

How to set up and validate locally

Check pages with Labels, e.g.

• Issue: http://gdk.test:3000/flightjs/Flight/-/issues/3
• Issues List: http://gdk.test:3000/flightjs/Flight/-/issues
• MR: http://gdk.test:3000/flightjs/Flight/-/merge_requests/4

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #444717 (closed), gitlab-ui!3306 (merged), &12868