In 3.0.0 we faced the bug
OPT-3620 that was not detected upfront by existing upgrade tests as these only cover the upgrade from previous to the current version.
To prevent this in future we should setup a schema integrity check into our upgrade/migration stage.
The goal is that the schema created by upgrading from the previous to the current version should be identical to the one created by booting up the current version on a fresh elasticsearch cluster
- a schema integrity IT is setup in the `migration` or `rolling upgrade` stage of our branch builds