Adjust rubocop rule for not valid FKs

What does this MR do?

This MR adjusts the rubocop rule for add_foreign_key. Normally we avoid using add_foreign_key since it validates the constraint within the same transaction. Instead of add_foreign_key we suggest to use add_concurrent_foreign_key. There is an exception where add_foreign_key is safe to use: adding a not valid FK.

If the FK definition contains validate: false the rubocop rule won't register an offense.

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team