ZenTao Integration menu item should be nested under Issues

Upstream Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/345551

Upstream MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/83367

Proposal

Zentao Integration menu item should be nested under Issues.

See Pajamas documentation: https://design.gitlab.com/regions/navigation#sub-menu-items

Current implementation	Expected result