Updates work items FF details and flag notes in Tasks page

What does this MR do and why?

Updates feature flag details related to work_items_mvc and work_items_beta feature flags.

Updates the history text and flag notes for the following topics in the Tasks page:

• Set a start and due date
• Add a task to a milestone
• Add a task to an iteration
• Lock discussion
• Two-column layout

References

Relates to: https://gitlab.com/gitlab-org/technical-writing/-/issues/1170