Update docs to disable rule in DB guides example

What does this MR do?

In the example for "Creating a new table when we have two foreign keys" we recommend using add_foreign_key - https://docs.gitlab.com/ee/development/migration_style_guide.html#examples.

We also have a Rubocop rule that warns against its usage - https://gitlab.com/gitlab-org/gitlab/-/blob/master/rubocop/cop/migration/add_concurrent_foreign_key.rb. This can cause confusion, recent example here - !41785 (comment 411886303).

This updates the examples too disable this rule. Alternatively we can start recommending add_concurrent_foreign_key to be used everywhere, even if the table is newly created and empty.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

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• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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