MINOR: Fix migration system tests for 3.6

[mumrah]

As part of validating 3.6.0 RC0, I ran the ZK migration system tests at the RC tag. Pretty much all of them failed due to recent changes (particularly, disallowing migrations with JBOD). All of the changes here are test fixes, so not a release blocker.

================================================================================
SESSION REPORT (ALL TESTS)
ducktape version: 0.11.3
session_id:       2023-09-19--007
run time:         8 minutes 51.147 seconds
tests run:        5
passed:           5
flaky:            0
failed:           0
ignored:          0
================================================================================
test_id:    kafkatest.tests.core.zookeeper_migration_test.TestMigration.test_online_migration.roll_controller=False
status:     PASS
run time:   2 minutes 56.029 seconds
--------------------------------------------------------------------------------
test_id:    kafkatest.tests.core.zookeeper_migration_test.TestMigration.test_online_migration.roll_controller=True
status:     PASS
run time:   3 minutes 1.037 seconds
--------------------------------------------------------------------------------
test_id:    kafkatest.tests.core.zookeeper_migration_test.TestMigration.test_pre_migration_mode_3_4.metadata_quorum=ISOLATED_KRAFT
status:     PASS
run time:   44.101 seconds
--------------------------------------------------------------------------------
test_id:    kafkatest.tests.core.zookeeper_migration_test.TestMigration.test_reconcile_kraft_to_zk
status:     PASS
run time:   1 minute 25.509 seconds
--------------------------------------------------------------------------------
test_id:    kafkatest.tests.core.zookeeper_migration_test.TestMigration.test_upgrade_after_3_4_migration
status:     PASS
run time:   43.993 seconds
--------------------------------------------------------------------------------

[showuon]

LGTM! Confirmed all the zookeeper_migration_test tests are passed after this change.

[showuon]

@mumrah , are you going to create a PR against the trunk branch?

[mumrah]

@showuon looks like some of the KIP-919 work is causing at least one of these tests to fail on trunk, so I'll need to investigate a bit. But yes, I'll port these fixes to trunk