Add Settings migration for require SHA when merging

Safwan Ahmedrequested to merge
saf-add-settings-migrations-to-support-require-sha-for-merge into master

What does this MR do and why?

This MR adds migrations for a cascading settings column, to support SHA parameter requirement when using the merge API see: gitlab-org#20551+ This is being made configurable to not cause a breaking API change, and give users opportunity to opt-in.

References

https://gitlab.com/gitlab-org/gitlab/-/work_items/599954+

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