Add Confidentiality Checkbox to New Test Case Form

What does this MR do and why?

Relates to #422121 (closed)

As part of &11229 (closed), we will be adding support for creating and editing confidential test cases in the UI.

While test cases will be migrated to the work items framework mid-next year, this is an interim solution to help enable the Quality team to manage test cases within a single source of truth for ongoing confidential projects.

This MR adds a confidentiality checkbox to the create new test case form that, when checked, will save a test case as confidential, similar to issues and epics.

Note: The associated backend MR to allow the CreateTestCase mutation to support a confidential attribute can be found here: !129871 (merged)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

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

1. Check out the branch for this MR to your local machine
2. Ensure you have an Ultimate license available on your GDK
3. Create a new group and project
4. In the project, go to Build > Test cases
5. Click on the "New test case" button
6. Create new test cases both with and without the Confidentiality checkbox checked
   1. Verify the test case saves as non-confidential when checkbox is unchecked
   2. Verify the test case saves as confidential when checkbox is checked

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.