Add a note about using BIGINT FKs

Leonardo da Rosarequested to merge
document-usage-of-bigint-foreign-keys into master

What does this MR do and why?

It adds a note about using BIGINT as default for all newly created FKs. https://docs.gitlab.com/ee/development/database/foreign_keys.html

Related gitlab-org/database-team/team-tasks#18 (closed)

Edited by Leonardo da Rosa

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