add Scala 3.0.0-RC2 and JDK 16, drop M3 and 15; move to sbt 1.5#503

SethTisue merged 2 commits intoscala:mainscala/scala-xml:mainfrom

SethTisue:scala-3.0.0-RC2SethTisue/scala-xml:scala-3.0.0-RC2Copy head branch name to clipboard