Fix Rake/Require offence in migrate rake task

What does this MR do and why?

This MR moves all requires into task definition of migrate (lib/tasks/gitlab/artifacts/migrate.rake) rake task.

Contributes to: #391459 (closed)

Screenshots or screen recordings

How to set up and validate locally

rake gitlab:artifacts:migrate

bundle exec rubocop lib/tasks/gitlab/artifacts/migrate.rake
Inspecting 1 file
.

1 file inspected, no offenses detected

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