codeintel: Update indexability heuristics

Created by: efritz

Before we had a minimum threshold on search and precise code intelligence events as well as the ratio of search to total events. This means that once the ratio threshold is no longer met as there's a precise index, we will no longer update that repository and the index will eventually fall out of sync.

Now, we have a condition to begin indexing (minimum search events + minimum ratio) and a condition to continue indexing (minimum precise events). This should help maintain up-to-date indexes for repositories that have begun to be indexed and are continually used. If they are not used, then they will fall below the threshold ratio and be indexed as "fresh" again.

Closes https://github.com/sourcegraph/sourcegraph/issues/11624.