NIFI-9399 Apply Secure Processing to TransformXml XSLT

[exceptionfactory]

Description of PR

NIFI-9399 Updates the TransformXml Processor to apply the Secure Processing property to configured XSLT sources.

The implementation uses XmlUtils.createSafeReader() to wrap the XSLT StreamSource in an XMLStreamReader configured with external entity resolution disabled. Updated unit tests verify intended behavior for both input XML and configured XSLT sources. Additional changes include updating Saxon HE from 9.6.0-5 to 10.6, and removing the unused XsltValidator private static class.

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