upgrades: exercise TestPauseMigration for secondary tenants #107393
Description
For context, the test TestPauseMigration checks that ongoing version upgrades can be paused and that if an upgrade is requested concurrently with a pause, it will be paused too.
As currently written, this test only works in the system tenant. It should have a variant that also works for secondary tenants.
Informs #76378
Jira issue: CRDB-30018
Epic: CRDB-48526