Update sbt-mdoc to 2.6.1 (#632)

Co-authored-by: eikek-scala-steward <[email protected]>

Author: eikek-scala-steward[bot]

project/plugins.sbt

@@ -4,5 +4,5 @@ addSbtPlugin("com.eed3si9n" % "sbt-buildinfo" % "0.12.0")
 addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.5.12")
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.3.2")
 addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.17.0")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.5.4")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.6.1")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.2")