Drop the sources column from the repo table

Created by: asdine

https://github.com/sourcegraph/sourcegraph/pull/13176 introduces a new table called external_service_repos that is used to denormalize the content of the sources column of the repo table. Dropping that column while migrating the data to the new table is taking too long when the repo table is big, this caused the migration to timeout on Sourcegraph cloud. We decided instead to keep that column in 3.20, make it read-only with a trigger, and remove it in 3.21.