Revisit handling of suspended tenants within Upgrade Tasks

Handling of suspended tenants with Upgrade Tasks should be revisited by migrations, and expected behavior of the process / documentation clarified so as to either prevent or preempt potential future failed upgrade tasks & associated HOTs.

See https://ops.internal.atlassian.com/jira/browse/PIR-16427 and https://ops.internal.atlassian.com/jira/browse/HOT-105426 for further details on an incident in which suspended tenant behavior was incongruent with potential upgrade task behavior.