search: enable repository ranking by default

Created by: keegancsmith

We currently only have reordering result by repository rank enabled on Sourcegraph.com. This switches it on with the same queue depth we use for sourcegraph.com. The queue depth is a heuristic we have tuned which balances latency with the number of backends you have.

Sourcegraph.com is our biggest known cluster (by far), so this default value is likely larger than needs be. However, since this code was introduced we have fixed issues around not receiving any updates from a backend. We now regularly send updates to the queue from backends, even if no results are found. So I believe we don't need to be as conservative with this value as in the past. So I believe this value is safe for customers. As a follow-up we should try tuning this value higher for sourcegraph.com and get a bit more stability in ranking.

Fixes https://github.com/sourcegraph/sourcegraph/issues/29729