Don't drop in DropAndReaddHasExternalWikiInProjects

What does this MR do?

This MR fixes the migration in question so it doesn't first drop and then re-add a column.

Are there points in the code the reviewer needs to double check?

Not really.

Why was this MR needed?

The old migration could lead to application errors as the column would temporarily not exist between the drop column and add column statements.

What are the relevant issue numbers?

https://gitlab.com/gitlab-com/infrastructure/issues/239

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together