Fix API docs slow migration issue
Created by: slimsag
This PR reintroduces #25994, which was reverted last night in #26074 as part of an incident in which the migration was too slow to succeed on Sourcegraph.com.
The actual migration was fine, and hence passed dev/CI/staging environments, but due to the scale of data on Sourcegraph.com was too slow to succeed there before Kubernetes killed the pod due to health checks.
The underlying cause was a single statement taking 4 minutes:
UPDATE lsif_data_documentation_pages SET search_indexed='false';
In this PR, we replace that statement instead with:
ALTER TABLE lsif_data_documentation_pages DROP COLUMN search_indexed;
ALTER TABLE lsif_data_documentation_pages ADD COLUMN search_indexed boolean DEFAULT 'false';
This is almost instantaneous, I have verified so on a clone of our production DB (thanks @efritz for the idea of testing this like this!)
Helps #21938