Update sbt-scalafix to 0.9.21

Author: scala-steward

project/plugins.sbt

@@ -3,7 +3,7 @@ addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "1.0.0")
 addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.2.0")
 addSbtPlugin("org.scala-native" % "sbt-scala-native" % "0.3.9")
 addSbtPlugin("ch.epfl.lamp" % "sbt-dotty" % "0.4.2")
-addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.20")
+addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.9.21")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" %  "2.4.2")
 addSbtPlugin("org.xerial.sbt" % "sbt-sonatype" % "3.9.4")
 addSbtPlugin("com.jsuereth" % "sbt-pgp" % "2.0.1")