Update sbt-mdoc to 2.9.0

Co-authored-by: kenji yoshida <6b656e6a69@gmail.com>

Author: scala-steward

project/plugins.sbt

@@ -7,6 +7,6 @@ addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject"      % "1.3.2")
 addSbtPlugin("org.scala-native"   % "sbt-scala-native"              % "0.5.11")
 addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "1.3.2")
 addSbtPlugin("org.scalameta"      % "sbt-scalafmt"                  % "2.6.0")
-addSbtPlugin("org.scalameta"      % "sbt-mdoc"                      % "2.4.0")
+addSbtPlugin("org.scalameta"      % "sbt-mdoc"                      % "2.9.0")
 
 scalacOptions += "-deprecation"