[refactor] Simplify startup code

[dizzzz]

It started with the idea to remove all old code code from jetty, as the classpath is now constructed in a different way.

The PR evolved into a significant rewrite refactoring and cleanup of the original code; Move methods to locations where used, combine functionality in a new Class, split-up the "doIt" method, with readable methods.

Summary

• classloader code is moved to org.exist.start.classloader.
• LatestFileResolver is now moved to the webstart code in org.exist.webstart
• LatestFileResolver has been cleaned up, uses a logger, made code more readable.
• Main
• rename runEx(runArgs) to 'startExistdb(runArgs)` - characters are free these days
• revised CompatibleJavaVersionCheck as the buggy Java15's are not relevant anymore - code remains for future usage.
• made CompatibleJavaVersionCheck code more readable
• significant rewrite of org/exist/start/Main.java to make it more readable, less error prone to see the flow etc.

[duncdrum]

Javadoc of all things is failing

[line-o]

This looks amazing 🤩
Could you walk us through the changes tonight?

[dizzzz]

The open questions I have

• shall I make this module start.jar compatible with Java8 ?
  • pro: decent handling of old java versions, with good error message
  • con: bin/startup.sh contains parameters that will stop java8 anyway.
• start.jar has no external dependencies
  • is that still relevant @wolfgangmm
• OK to remove all all old jetty-codebase references?
  • all has been removed/refactored and is eXist-db specific

[dizzzz]

I tried to simplify the startup of appbundler by removing these xml files

• (old) maven plugin crashes with NPE, cant find reason, is under documented
  • great work of Adam understanding the plugin config
• idea is to create a (python)script/java module that can update the appbundler xml files
  • this enables updating/changing jar files after release

[duncdrum]

@dizzzz

• I don't see much of a point in J8 in this instance, it ll will error one way or the other
• My guess for relevance of 0 deps would be yes, but @wolfgangmm knows more
• uhmm yes please, its a new dawn ;-)