Start using new table for instance integrations (using feature flag)

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As part of &14638 at this point we should have identical amount of records of instance integrations in integrations table and mirroring records in instance_integrations tables, which should be ready to perform a swap of using integrations to instance_integrations.

Proposed solution

1. Update existing usages of instance integrations that use integrations records to use instance_integrations
2. Add a feature flag to swap to the new table records