KAFKA-18280:fix e2e TestSecurityRollingUpgrade.test_rolling_upgrade_sasl_mechanism_phase_one

[TaiJuWu]

JIRA: https://issues.apache.org/jira/projects/KAFKA/issues/KAFKA-18280

In KRaft mode, miniKDC will start two services so client side token and server side token is unmatched after node restart.

In this patch, we do not unassign MiniKDC to avoid creating another service.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[TaiJuWu]

Result

SESSION REPORT (ALL TESTS)
ducktape version: 0.12.0
session_id:       2024-12-23--021
run time:         4 minutes 12.166 seconds
tests run:        1
passed:           1
flaky:            0
failed:           0
ignored:          0
================================================================================
test_id:    kafkatest.tests.core.security_rolling_upgrade_test.TestSecurityRollingUpgrade.test_rolling_upgrade_sasl_mechanism_phase_one.new_client_sasl_mechanism=PLAIN.metadata_quorum=ISOLATED_KRAFT
status:     PASS
run time:   4 minutes 11.980 seconds
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