Update sbt-mdoc to 2.2.21 (#221)

Author: scala-steward

project/plugins.sbt

@@ -8,7 +8,7 @@ addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.4.2")
 addSbtPlugin("com.codecommit" % "sbt-spiewak-sonatype" % "0.20.4")
 addSbtPlugin("com.codecommit" % "sbt-github-actions" % "0.9.5")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.4.2")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.2.20")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.2.21")
 addSbtPlugin("io.github.jonas" % "sbt-paradox-material-theme" % "0.6.0")
 addSbtPlugin("com.typesafe.sbt" % "sbt-site" % "1.4.1")
 addSbtPlugin("com.typesafe.sbt" % "sbt-ghpages" % "0.6.3")