Drop workspaces column dns_zone

Issue - Backend: Drop dns_zone from workspace table (#480656 - closed)

What does this MR do and why?

Following the doc here, https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-columns

Now with previous MR, we add ignored_column on dns_zone, and also updated all reference on this from workspace.dns_zone to workspace.workspaces_agent_config.dns_zone in release 17.5

now we will drop the column in release 17.6

MR acceptance checklist

• remove dns_zone from workspace table with migration

Screenshots or screen recordings

There should be no impact on this MR, as the ignored_column on dns_zone already exists on workspaces model.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.