Rewrite code of EndnoteXmlImporter avoid JAXB

[koppor]

The code of org.jabref.logic.importer.fileformat.EndnoteXmlImporter is well-written, but relies on JAXB. It should be rewritten using StAX-Parser and thus getting rid of a JAXB dependency.

The code reads like we do not hit polymorphism in XMLElements (which is not supported by JAXB. See FasterXML/jackson-modules-base#127).

Example method:

/src/main/java/org/jabref/logic/importer/fileformat/EndnoteXmlImporter.java#L177

    private BibEntry parseRecord(Record endNoteRecord) {

See /src/main/java/org/jabref/logic/importer/fileformat/MedlineImporter.java#L52 for some code hints.

Similar to #9673, the gradle build target generateEndnoteSource can be removed and the xsd file be removed from the source repository.

Negleted Implementation Option

Thus, it seems to be "just" to include the dependency to jackson module jakarta-xmlbind and to use Jackson's XMLMapper. Code examples are at https://stackify.com/java-xml-jackson/.