Skip re-creating of constraint when migration spec is skipped

What does this MR do and why?

Skip re-creating of constraint when migration spec is skipped

References

• gitlab-org/quality/engineering-productivity/master-broken-incidents#23929 (closed)
• https://gitlab.com/gitlab-org/gitlab/-/jobs/13942390053

How to set up and validate locally

This is a time-based failure so this also fails locally:

bundle exec rspec spec/lib/gitlab/background_migration/backfill_slack_integrations_sharding_key_spec.rb

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.