Fix migration to make it fast

What does this MR do?

Changing the migration introduced in !5387 (merged) so that it's fast. The idea is that the content of the projects.has_external_wiki is currently wrong, so we can just drop and recreate the column.

Does this MR meet the acceptance criteria?

• No CHANGELOG needed
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together