Fix deprecated ScalaInstance.libraryJar

[ashawley]

Change to using xsbti.compile.ScalaInstance.libraryJars in sbt build.

[info] Loading project definition from scala-xml/project
scala-xml/build.sbt:121: warning: method libraryJar in trait ScalaInstance is deprecated: see corresponding Javadoc for more information.
      scalaInstance.value.libraryJar
                          ^

[ashawley]

I spent an hour trying to figure this out. Hopefully, the "corresponding Javadoc" is published somewhere for xsbti.compile.ScalaInstance? I couldn't find it. I had to just grep through GitHub repos.

[ashawley]

These warnings in JDK 11 and later already existed:

[warn] /home/circleci/project/shared/src/main/scala/scala/xml/package.scala:11:1: Could not find any member to link for "javax.xml.parsers.SAXParser".
[warn] /**
[warn] ^
[warn] /home/circleci/project/shared/src/main/scala/scala/xml/parsing/FactoryAdapter.scala:188:3: Could not find any member to link for "org.xml.sax.SAXException".
[warn]   /**
[warn]   ^
[warn] /home/circleci/project/shared/src/main/scala/scala/xml/include/XIncludeException.scala:13:1: Could not find any member to link for "java.lang.Throwable#getMessage".
[warn] /**
[warn] ^
[warn] /home/circleci/project/shared/src/main/scala/scala/xml/include/sax/EncodingHeuristics.scala:38:3: Could not find any member to link for "java.io.IOException".
[warn]   /**
[warn]   ^