Fix migrations bug through overriding ConvertToBigint

What does this MR do and why?

Fix for #2718 (closed)

Let JH's pipeline (test environment) also execute the migration in MR:

• https://gitlab.com/gitlab-org/gitlab/-/merge_requests/114441/diffs
• https://gitlab.com/gitlab-org/gitlab/-/merge_requests/115544/diffs

This will ensure that the structure.sql and migrations are consistent in the JH pipelines (test environment).

Screenshots or screen recordings

How to set up and validate locally

Start Rails console in GDK env:

include Gitlab::Database::MigrationHelpers::ConvertToBigint

puts com_or_dev_or_test_but_not_jh?
# true

MR acceptance checklist

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