[Legacy Nav] Convert Admin Sidebar from HAML to Panel

We should convert our old HAML driven sidebar code:

• https://gitlab.com/gitlab-org/gitlab/-/blob/ad056d993207aae2e299cd0ce6dbd62183462d93/app/views/layouts/nav/sidebar/_admin.html.haml

to something like this

• https://gitlab.com/gitlab-org/gitlab/-/blob/ad056d993207aae2e299cd0ce6dbd62183462d93/lib/sidebars/groups/panel.rb

This will help to convert it to the super sidebar as well and can be well done with regression tests.