As part of the namespace table migration to support schemas, we changed
the name in the old namespace table's descriptor from "namespace" to
"namespace_deprecated". However, we only made this change for new
clusters, not clusters that were migrated from 19.2. This resulted in
breaking backup on migrated clusters, since it expects that table names
are unique.

I supplemented our namespace table migration to retrieve the deprecated
namespace table's descriptor and update its name as desired. I also
updated the existing migration to use Puts rather than CPuts since it
should be fine to overwrite the existing KVs.

Fixes #43979

Release note: None