Update sbt-scalajs, scalajs-compiler, ... to 1.21.0 in series/1.x

Author: typelevel-steward[bot]

project/plugins.sbt

@@ -1,4 +1,4 @@
 addSbtPlugin("org.typelevel" % "sbt-typelevel" % "0.8.5")
-addSbtPlugin("org.scala-js"  % "sbt-scalajs"   % "1.20.2")
+addSbtPlugin("org.scala-js"  % "sbt-scalajs"   % "1.21.0")
 addSbtPlugin("org.scala-native"   % "sbt-scala-native" % "0.5.10")
 addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "1.3.2")