Deprecate Xalan for removal

[vlsi]

JMeter ships with Xalan, however, many its features are available with Java.
Xalan is not actively maintained, so we should consider removing Xalan APIs.

Unfortuantely, JMeter public APIs include Xalan classes, so we can't easily drop it.

The plan is as follows:

1. Deprecate Xalan for removal in JMeter 6.0 <-- this issue
2. Remove Xalan somewhere in 2026-2027

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Looks like org.apache.jmeter.util.XPathUtil is not used by third-party code:
https://github.com/search?q=%22import+org.apache.jmeter.util.XPathUtil%22&type=code

org.apache.jmeter.util.PropertiesBasedPrefixResolver is not used either:
https://github.com/search?q=%22import+org.apache.jmeter.util.PropertiesBasedPrefixResolver%22&type=code

Looks like we could just replace the implementation and drop Xalan