Use labels widget on epic board and MR sidebar [RUN AS-IF-FOSS]

What does this MR do and why?

Use new labels widget in epic board sidebar and merge request sidebar.

Behind :labels_widget feature flag.

Screenshots or screen recordings

Merge Request sidebar	Epic board sidebar

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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 #343606 (closed)