Prepare system_note_metadata for bigint migration

What does this MR do and why?

In preparation of Swap system_note_metadata.note_id to bigint (!117343 - merged) we need to validate FK asynchronously as it takes almost 4 hours to validate.

Async FK validation docs - https://docs.gitlab.com/ee/development/database/add_foreign_key_to_existing_column.html#validate-the-foreign-key-asynchronously.

See #392815 (closed).

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