Enable self-managed UI based bulk assignment feature by default

What does this MR do and why?

Relates to: https://gitlab.com/gitlab-org/gitlab/-/issues/443569

The bulk assignment service for self-managed instances have been tested through E2E and manual testing and they have passed the expectations. The feature flag is now ready to be enabled by default, which can be added to the next release package.

Note: This MR adds a new entry to the changelog which is supposed to be automatically generated according to the documentation

Test results: https://gitlab.com/groups/gitlab-org/-/epics/12959#note_2040347612

Release deadline: 2024-08-12