Update sbt-mdoc to 2.9.0 (#804)

Author: scala-steward

project/plugins.sbt

@@ -10,7 +10,7 @@ addSbtPlugin("org.scoverage" % "sbt-coveralls" % "1.3.15")
 // This sets the 'version' property based on the git tag during release process to publish the right version
 addSbtPlugin("com.github.sbt" % "sbt-dynver" % "5.1.1")
 
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.7.2")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.9.0")
 
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.6.0")