NN running newer software rejects loading the fsimage during rolling upgrade.

During rolling upgrade, the standby NN that has a newer layout version will complain about that the fsimage is too old, and require starting the NN using the -upgrade option:

File system image contains an old layout version -55. An upgrade to version -56 is required.

However, -upgrade is disabled during rolling upgrade, thus it is impossible to restart the standby NN:

org.apache.hadoop.hdfs.protocol.RollingUpgradeException: Failed to upgrade namenode since a rolling upgrade is already in progress. Existing rolling upgrade info:
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