Group minimum role should be Guest for adding child epic

Currently, in order to add a child epic to a parent, the user must have a minimum role of Reporter in the parent epic. Since Guest users can view epic data, there is no concern of leaking data by allowing this behavior.

In this issue, we will change the group minimum role requirements to Guest.

Note: The policy :admin_epic_relation added in !106415 (merged) should be used to replace :admin_epic.

Tasks

• Update documentation !106415 (merged)
• Update services and endpoints !106415 (merged)
• Update quick actions !106558 (merged)
• Update UI dropdown for epics tree !106434 (merged)

Docs Support

For issues requiring an update to GitLab docs, please note that this is part of the definition of done for this issue. Please create a separate MR for the docs update related to the MR for the actual change to ensure code reviews are not blocked by TW reviews.

• This issue requires an update to GitLab docs