Support Java 21 on Scala.js

[ekrich]

Scala.js does not work for Scala 3.3.x. Classes extending java.util.SequencedCollection cause problems. Note that Scala Native has the class added although it is just a stub at this time.

Example error:

[info] Fast optimizing /Users/eric/workspace/sconfig/sconfig/js/target/scala-3.3.3/sconfig-test-fastopt
[error] Referring to non-existent class java.util.SequencedCollection
[error]   called from org.ekrich.config.impl.SimpleConfigList
[error]   called from org.ekrich.config.impl.ConfigValueSharedTest.valuesToString()void
[error]   called from org.ekrich.config.impl.ConfigValueSharedTest$scalajs$junit$bootstrapper$.invokeTest(java.lang.Object,java.lang.String)scala.concurrent.Future
[error]   dispatched from org.scalajs.junit.Bootstrapper.invokeTest(java.lang.Object,java.lang.String)scala.concurrent.Future
[error]   called from private org.scalajs.junit.JUnitTask.$anonfun$executeTestMethod$7(org.scalajs.junit.Bootstrapper,java.lang.Object,org.scalajs.junit.TestMetadata)scala.concurrent.Future