Group minimum role should be Guest for work item relations
## Summary
As per https://gitlab.com/groups/gitlab-org/-/epics/10266, we should reduce the minimum role to relate two records to `Guest`. This epic is to address the relationships associated with work items.
## Proposal
| Relation | Description | {-Current Permissions-} | {+New Permissions+} | Status |
| ------ | ------ | ------ | ------ | ------ |
| Objective-Objective (vertical) | Add a child objective to an objective | `Reporter` access in both work items |`Guest` access in both work items | Scheduled with https://gitlab.com/gitlab-org/gitlab/-/issues/404795 |
| Objective-Key Result (vertical) | Add a key result to an objective | `Reporter` access in both work items | `Guest` access in both work items | Scheduled with https://gitlab.com/gitlab-org/gitlab/-/issues/404796 |
epic