[Self-managed] Deactivation of dormant users
## Summary
Identifying dormant users and deactivating (SM)/removing (SaaS) them is a labor intensive and not very intuitive process for our customers. We should make user management, specifically around dormant users, easier so admins/owners can focus their time on less arduous tasks and budgets are not accidentally blown up.
## Iterations
1. [ ] Introduce indicator that a user has had inactivity for 90 days or more https://gitlab.com/groups/gitlab-org/-/epics/4440
1. [x] Provide an admin setting to automate deactivation of dormant users (Self-managed)
1. BE: https://gitlab.com/gitlab-org/gitlab/-/issues/211754
1. UX: https://gitlab.com/gitlab-org/gitlab/-/issues/282537
1. FE: https://gitlab.com/gitlab-org/gitlab/-/issues/320875
1. [ ] Provide an owner setting to automate removal of dormant users (Saas) https://gitlab.com/groups/gitlab-org/-/epics/7533
## Future Iterations
1. [ ] Allow users to set the inactivity window with a minimum of 90 days https://gitlab.com/gitlab-org/gitlab/-/issues/336747
1. [ ] Create a companion feature in SaaS (remove member, not deactivate) https://gitlab.com/groups/gitlab-org/-/epics/7533
epic