Only allow Advanced Search limiting for top level namespaces

Background

Index integrity is currently tied to users performing search results. It would be better to move the work to a Cron Job. To do this, we need the ability to query all indexed projects which isn't possible due to the use of indexing limiting and allowing any namespace type (top level group and subgroup) to be limited. The queries to get all indexed projects time out. It would be possible to iterate through all indexed projects if top level namespaces were the only types allowed for indexing limiting.

This would be a potential breaking change for customers who are limiting Advanced Search indexing for non-top level namespaces

Proposal

The first step is to gather self managed usage data

Only allow top level namespaces to be added as limited namespaces for Advanced Search. Technical implementation plan TBD