Make Jira Issue menu items sub-menu items of the Issues top-level menu item

Raised in !69602 (comment 695590228).

Pajamas documentation suggests not to override top-level navigation items. See https://design.gitlab.com/regions/navigation#sub-menu-items for more context.

Current Situation:

Issues
- List 
- Boards
- Service Desk
- Milestones
- Iterations
- Requirements

Jira issues 
- Issue list
- Open Jira

Desired Situation

Issues
- List 
- Boards
- Service Desk
- Milestones
- Iterations
- Requirements
------------
- Jira issues 
- Open Jira