Show placeholder user reassignment deadline and disable undo action after expiration
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=576117) </details> <!--IssueSummary end--> ## 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
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