Use new 'ROLE_of' helpers in all Workspaces domain code

MR: Pending

Description

The following discussion from !175486 (merged) should be addressed:

@pedropombeiro started a discussion: (+2 comments)

suggestion: can we simplify this to:

    # 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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