upgrades: exercise TestConcurrentMigrationAttempts with secondary tenants

[knz]

For context, the test TestConcurrentMigrationAttempts checks that multiple concurrent issues of SET CLUSTER SETTING version make progress in each client that issues it.

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-30019

Epic: CRDB-48526