Update sbt-scalajs, scalajs-compiler, ... to 1.21.0 in series/3.x

Author: typelevel-steward[bot]

project/plugins.sbt

@@ -2,7 +2,7 @@ libraryDependencies += "org.scala-js" %% "scalajs-env-selenium" % "1.1.1"
 
 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("pl.project13.scala" % "sbt-jmh" % "0.4.8")
 addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.8.2")