Update sbt-unidoc to 0.6.0 (#1951)

Author: scala-steward

project/plugins.sbt

@@ -12,7 +12,7 @@ addSbtPlugin("org.scala-native" % "sbt-scala-native" % "0.5.8")
 
 addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "1.1.4")
 
-addSbtPlugin("com.github.sbt" % "sbt-unidoc" % "0.5.0")
+addSbtPlugin("com.github.sbt" % "sbt-unidoc" % "0.6.0")
 
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.4")