[Feature flag] Enable group_multi_select_tokens

What does this MR do and why?

Enable group_multi_select_tokens by default.

It is enabled globally on .com currently.

Maybe changelog could be more clear. Suggestion:

Group search tokens for labels and assignees

Changelog: changed

How to set up and validate locally

1. Visit any issue list page
2. Filter by labels or assignees
3. See checkboxes in dropdown, and results are grouped in the same little box

Related to #428741