Update sbt-example to 9.0.0 (#474)

Author: scala-steward

project/plugins.sbt

@@ -8,7 +8,7 @@ addSbtPlugin("com.github.sbt" % "sbt-pgp" % "2.1.2")
 
 addSbtPlugin("org.lyranthe.sbt" % "partial-unification" % "1.1.2")
 
-addSbtPlugin("com.thoughtworks.example" % "sbt-example" % "8.1.0")
+addSbtPlugin("com.thoughtworks.example" % "sbt-example" % "9.0.0")
 
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.1.0")