Enhancement: Remove the notice that app-only permissions are not supported on commands dealing with event-based retention.

An update to the Microsoft Graph, the retention endpoints have been updated. They now support app-only permissions. We can remove the notice that says app-only permissions aren't supported for these commands:

- m365 purview retentionevent [verb]
- m365 purview retentioneventtype [verb]
- m365 purview retentionlabel [verb]