Update sbt-mdoc to 2.9.0

Author: typelevel-steward[bot]

project/plugins.sbt

@@ -5,6 +5,6 @@ addSbtPlugin("org.scala-native" % "sbt-scala-native" % "0.5.10")
 addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "1.3.2")
 addSbtPlugin("com.armanbilge" % "sbt-scala-native-config-brew-github-actions" % "0.4.0")
 addSbtPlugin("io.github.sbt-doctest" % "sbt-doctest" % "0.12.3")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.8.2")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.9.0")
 
 libraryDependencySchemes += "com.lihaoyi" %% "geny" % VersionScheme.Always