Discuss how we handle records with deleted users in the workspaces domain
## Description
Spin off from this thread https://gitlab.com/gitlab-org/gitlab/-/merge_requests/179604#note_2322858289. We are trying to set a precedent on how we handle records that hold references to users who can be deleted. The current options are:
- cascade delete (delete the record that the user is associated with)
- nullify the user in a column of associated records
- use the ghost user as a placeholder
All are viable options depending on the context, but we currently opt into the nullify approach in handling cluster agent mappings. We should be able to come up with a guided path to determine what option to use in any given context, to be consistent and set a precedent.
<!-- TODO: What kind of issue is this? Please uncomment and include one of the following:
/label ~"type::feature"
/label ~"type::bug"
/label ~"type::maintenance"
-->
<!-- OPTIONAL: Is this a feature/enhancement?
- Please describe the impact this feature/enhancement will have on the user experience and/or the product as a whole.
- Provide a user story to illustrate the use case for this feature/enhancement. Include examples to help communicate the intended functionality.
-->
<!-- OPTIONAL: Is this a bug? Please uncomment and include the following:
- **Current behavior:**
- **Expected behavior:**
- **Steps to reproduce:**
- **Additional information:** please include VSCode version, browser dev tool logs, etc that might be helpful to debug the issue!
-->
## Acceptance criteria
- [ ] Product aligned guidelines on picking an approach on handling deleted users
## Implementation plan
<!-- DO NOT TOUCH BELOW -->
issue