Show placeholder user reassignment deadline and disable undo action after expiration
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## Background
As part of the data retention policy implementation https://gitlab.com/gitlab-org/gitlab/-/issues/527002, placeholder reassignment will only be allowed for 1 year. Group owners need visibility of this deadline and should be prevented from undoing keep-as-placeholder actions after the expiration period.
## Requirements
### UI Updates
- Add deadline information to the Placeholder User table showing when the reassignment period expires
- Disable the "Undo keep placeholder" button when the reassignment period has expired
### Backend Changes
- Update GraphQL API to include deadline/expiration information
- Modify GraphQL mutation to prevent undoing keep-as-placeholder action after the expiration period
## Acceptance Criteria
- [ ] Deadline is visible in the Placeholder User table
- [ ] "Undo keep placeholder" button is disabled for expired placeholder users
- [ ] GraphQL API returns deadline information for placeholder users
- [ ] GraphQL mutation blocks undo actions for expired placeholder users
- [ ] Group owners can undo keep-as-placeholder actions only within the one-year period
issue