NIFI-9371 Remove synchronized keyword from Active Threads methods

[exceptionfactory]

Description of PR

NIFI-9371 Removes the synchronized keyword from methods in StandardProcessorNode that interact with the activeThreads ConcurrentHashMap. As a result of changes in #5509 for NIFI-9365, the ConcurrentHashMap provides synchronization around changes to the collection of active threads, making the synchronization unnecessary at the method level.

Additional changes include replacing several System.out.println() statements with SLF4J Logger.info() in nifi-system-test-suite classes.

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[markap14]

Thanks @exceptionfactory I agree with the changes, it makes sense to get rid of the synchronized keyword now that the Map was changed to a ConcurrentHashMap. +1 will merge.