Fix removeBillingFKWithCore migration location

[Weiko]

removeBillingFKWithCore migration is in the wrong folder and is not executed as intended.
Moving to billing folder to fix that and to be only run in billing mode

[greptile-apps]

PR Summary

This change relocates a billing-related migration file to ensure it only executes when running in billing mode, affecting foreign key constraints between billing and workspace tables.

• Migration file moved from /core/migrations/ to /core/migrations/billing/ directory to properly scope its execution
• Removes three foreign key constraints in billingEntitlement, billingSubscription, and billingCustomer tables that reference workspace.id
• Migration timestamp 1733753649142 indicates this should run after other billing table migrations

_{1 file(s) reviewed, no comment(s)
Edit PR Review Bot Settings | Greptile}