Add 'Security & Compliance' as top-level navigation item to the project-sidebar

What does this MR do?

This MR moves the 'Security Dashboard' link from the 'Project' tab into a newly created top-level item 'Security & Compliance'.

12250

Note: MR that updates related documentation can be found here https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31046

Before:

After:

Note: There is also a related MR for the CE.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry