[Scala] get build to work with Scala 3

[pjfanning]

• Code needs small hack for spliterator - won't compile in Scala 3 otherwise - possible Scala 3 compiler bug
• removes Scala 2.12 build option beause existing code uses Scala Factory class that was only added in Scala 2.13 - if you want to support Scala 2.12, new Scala 2.12 specific code is needed
• updates CI to use +test which tests with all the crossScalaVersions and gets rid of setup-scala action that is deprecated in favour of setup-java - sbt build tool installs the Scala version it needs anyway so no need to pre-install Scala

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass, see here for how to run them

[chaokunyang]

LGTM, thanks you @pjfanning.

[chaokunyang]

sbt +test for all scala version tests. new scala tips for me 😄