Move Productivity Analytics to the group level

Overview

With the creation of a group-level analytics menu in #195979 (closed), we can bring Productivity Analytics out of the top-level analytics menu and into the group-level.

Proposal

• Remove Productivity Analytics from the top-level Analytics workspace.
• Add Productivity Analytics to the group-level analytics sidebar menu.
• Remove the group selector from Productivity Analytics.
  • Instead of selecting the relevant group, the analytics presented in the charts should match the group (e.g. viewing https://gitlab.com/gitlab-org/-/analytics/productivity_analytics should display Productivity Analytics for the gitlab-org group).

Please keep this in Premium/Silver.