Rename default repos to indexable repos

Created by: tsenart

When we introduced the default_repos table we didn't have the repo.stars column, so we manually selected specific repos that should be indexed on sourcegraph.com. With the addition of repo.stars we can instead select all repos that have stars >= 20, which is our baseline for 5m indexed repos on dot com.

A follow up PR will drop the default_repos table and I will modify ghdump to skip the step of adding repos to it.