Update service desk email visibility role

What does this MR do and why?

What does this MR do?

Updates docs on Service Desk email visibility to account for the introduction of Planner role which was introduced in GitLab 17.7 New Planner user role (#482733 - closed)

Why?

Planner role was created so that customers can assign a sub-Reporter role which grants a user project management privileges, including the ability to view, create, and edit issues for projects.

Since the Planner role can view, create, and edit issues for a project, including confidential issues, it makes sense that Planner role is also allowed to see the internal email address which can be used to create issues in a project via service desk.

References

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