Add support for updating task confidentiality

What does this MR do and why?

This MR adds support for toggling Task Item's confidentiality.

Screenshots or screen recordings

Screen_Recording_2022-08-05_at_4.37.28_PM

How to set up and validate locally

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

1. To get this feature working you need to enable a bunch of feature flags on any project, commands to run in Rails console as given as follows;

Feature.enable(:work_items, Project.find_by_full_path('gitlab-org/gitlab-test'))
Feature.enable(:work_items_mvc_2, Project.find_by_full_path('gitlab-org/gitlab-test'))
Feature.enable(:work_items_hierarchy, Project.find_by_full_path('gitlab-org/gitlab-test'))
Feature.enable(:work_items_create_from_markdown, Project.find_by_full_path('gitlab-org/gitlab-test'))

2. Once all the feature flags are enabled, open any existing issue from the target project where flags were enabled.
3. You should see an option to create a child task, create one from there.
4. Click on the created task and it should open a modal as shown in screen recording above.

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.

Related to #368576 (closed), #368575 (closed)