Clarify error text for work item confidentiality hierarchy conflicts

What does this MR do and why?

The current error text when attempting to set a work item as confidential, while child items are not confidential, can be hard to follow in its current format.

This MR updates the copy to clarify the error and what needs to be addressed.

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Screenshots or screen recordings

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Before	After
A confidential work item cannot have a parent that already has non-confidential children	All child items must be confidential in order to turn on confidentiality.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a work item (non-confidential) with at least one non-confidential child items
   1. E.g. create an issue with tasks, change the route from /issues/ to /work_items/ to open the work item view
2. Try to enable confidentiality on the work item, an error should appear