NIFI-8304 Reduced test Jetty Server QueuedThreadPool to 8 threads

[exceptionfactory]

Description of PR

NIFI-8304 Introduces a configured QueuedThreadPool with a maximum of eight threads for running unit tests for InvokeHTTP. Multiple executions of the GitHub CI runner on JDK 11 succeeded after this change, as opposed to a number of recent failures in the SSL versions of the InvokeHTTP unit tests. The default minimum number of threads for Jetty is eight, so this change brings the maximum number in line with the minimum, reducing it from the default value of 200.

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