Point to compatibility version drift in migration docs

When migrating from self-managed CH to ClickHouse Cloud, some users notice a difference in behavior which can commonly be explained by different compatibility settings. In Cloud, the compatibility is set when a service is created and does not change (at least at the service level) unless explicitly requested (see Version compatibility). It'd be nice to add a note to migration guides to point this out.

See this thread in Community Slack for an example of user confusion.