Merge pull request #3864 from scala-steward/update/mdoc-2.9.0

Update mdoc, sbt-mdoc to 2.9.0

Author: mergify[bot]

project/plugins.sbt

@@ -8,7 +8,7 @@ addSbtPlugin("com.github.sbt" % "sbt-native-packager" % "1.11.7")
 addSbtPlugin("com.github.sbt" % "sbt-header" % "5.11.0")
 addSbtPlugin("org.typelevel" % "sbt-tpolecat" % "0.5.3")
 addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.4.8")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.8.2")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.9.0")
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.4.4")
 addSbtPlugin("org.typelevel" % "sbt-typelevel-mergify" % "0.8.5")