This commit updates the Saxon-HE dependency to version 11.4.

It refactors the Saxon-related code as follows:
- update the function+registration code to the latest API changes
- extract the Saxon schema reader factory (used by Jing) in its own class
- the Saxon schema reader factory is made available to Jing via the SPI
  mechanism (declared in
  `META-INF/services/com.thaiopensource.validate.SchemaReaderFactory`).
  This notably allows Jing to find and use our factory for both Schematron
  and NVDL.
- the Saxon schema reader factory no longer extends Jing's
  `NewSaxonSchemaReaderFactory`, which set the feature "XSLT_VERSION"
  to "2.0" and caused a Warning to be issued by Saxon 11.

Fix #1341