Update sbt-scalajs, scalajs-compiler, ... to 1.6.0 (#511)

Author: scala-steward

project/plugins.sbt

@@ -3,7 +3,7 @@ addSbtPlugin("io.spray" % "sbt-revolver" % "0.9.1")
 addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.2.22" )
 addSbtPlugin("com.geirsson" % "sbt-ci-release" % "1.5.5")
 
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.3.1")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.6.0")
 addSbtPlugin("ch.epfl.scala" % "sbt-scalajs-bundler" % "0.20.0")
 
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject"      % "1.1.0")