[fix][test] Fix flaky ClusterMigrationTest.testClusterMigrationWithReplicationBacklog

[merlimat]

Flaky test failure

java.lang.AssertionError: expected [false] but found [true]
	at org.testng.Assert.fail(Assert.java:110)
	at org.testng.Assert.failNotEquals(Assert.java:1577)
	at org.testng.Assert.assertFalse(Assert.java:78)
	at org.testng.Assert.assertFalse(Assert.java:88)
	at org.apache.pulsar.broker.service.ClusterMigrationTest.testClusterMigrationWithReplicationBacklog(ClusterMigrationTest.java:525)

Summary

• Fix flaky ClusterMigrationTest.testClusterMigrationWithReplicationBacklog by increasing the retry timeout for the replication backlog drain check from 10s to 30s.
• After restarting broker3, the replication backlog may take longer to drain due to reconnection and replication catchup time.

Documentation

• doc-not-needed
  (Your PR doesn't need any doc update)

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