Use new 'ROLE_of' helpers in all Workspaces domain code
MR: Pending
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## Description
Use new 'ROLE_of' helpers in all Workspaces domain code
The following discussion from !175486 should be addressed:
- [ ] @pedropombeiro started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175486#note_2269689181): (+2 comments)
> **suggestion**: can we simplify this to:
>
> ```suggestion:-12+0
> # create the minimum privileged user that should have the project and namespace
> # permissions to access the workspaces_agent_config.
> create(:user, developer_of: project.namespace)
> end
>
> let_it_be(:unauthorized_user) do
> # create the maximum privileged user that should NOT have the project and namespace
> # permissions to access the agent.
> create(:user, reporter_of: project.namespace)
> ```
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