Draft: Enables visible_label_selection_on_metadata feature flag

What does this MR do and why?

Enables visible_label_selection_on_metadata feature flag by default for 16.0.

Follow-up to Add improved label selection to issues and MRs ... (!88908 - merged)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a new issue -> http://gdk.test:3000/flightjs/Flight/-/issues/new
   1. Add labels
   2. They're now visible below the dropdown
2. Create a new MR -> e.g. http://gdk.test:3000/flightjs/Flight/-/merge_requests/new?merge_request%5Bsource_project_id%5D=6&merge_request%5Bsource_branch%5D=3-test-branch&merge_request%5Btarget_project_id%5D=6&merge_request%5Btarget_branch%5D=master
   1. Add labels
   2. They're now visible below the dropdown

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 Improve label selection (gitlab-org/gitlab-services/design.gitlab.com#1269 - closed), [Feature flag] Rollout of `visible_label_select... (#364534 - closed), UX Paper Cuts 16.0 → Labels and Lists (#398228 - closed)