Migrate usage of snowflake_user to snowflake_service_user and snowflake_legacy_service_user

[ian-r-rose]

Snowflake recently introduced the concept of "service users" and "legacy service users" as different flavors of users from human users. This mostly allows you to tag different user types and apply different password policies to them. As part of this, the Snowflake terraform provider introduced two new resource types snowflake_service_user and snowflake_legacy_service_user.

The users in our default terraform configuration are actually service users, so we should migrate from the old resource type to the new ones. For the most part, I think we want snowflake_service_user, except for the sentinel user, which needs to be a snowflake_legacy_service_user due to Sentinel being username/password only.