Update sbt-mdoc to 2.6.3 (#1814)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: scala-steward

project/plugins.sbt

@@ -24,6 +24,6 @@ addSbtPlugin("com.thesamet" % "sbt-protoc-gen-project" % "0.1.8")
 
 addSbtPlugin("com.eed3si9n" % "sbt-projectmatrix" % "0.10.1")
 
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.6.2")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.6.3")
 
 addSbtPlugin("org.scalameta" % "sbt-native-image" % "0.3.4")