chore(migrations) deprecate migration helpers (#6441)
chore(migrations) deprecate migration helpers (#6441)

### Summary

We have never documented this properly and we don't use it in any of the current
migrations. This was needed for pre `1.0.0` migrations.

With this deprecation, we have option to either:
1. remove it with `3.0.0`
2. if feedback demands that we need to keep it:
   then remove the warning and properly document it
Kong/kong,Kong/kong,Kong/kong