Merge pull request #630 from scala-steward/update/sbt-header-5.9.0

Update sbt-header to 5.9.0

Author: xuwei-k

project/plugins.sbt

@@ -3,4 +3,4 @@ addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.2.0")
 addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "1.2.0")
 addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.11.0")
 addSbtPlugin("org.scala-native" % "sbt-scala-native" % "0.4.7")
-addSbtPlugin("de.heikoseeberger" % "sbt-header" % "5.7.0")
+addSbtPlugin("de.heikoseeberger" % "sbt-header" % "5.9.0")