Start using new table for instance integrations (using feature flag)
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As part of https://gitlab.com/groups/gitlab-org/-/epics/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
issue