Update sbt-mdoc to 2.6.1

Author: scala-steward

project/plugins.sbt

@@ -2,5 +2,5 @@ addSbtPlugin("ch.epfl.scala"    % "sbt-bloop"                 % "1.4.6")
 addSbtPlugin("com.geirsson"     % "sbt-ci-release"            % "1.5.5")
 addSbtPlugin("com.github.cb372" % "sbt-explicit-dependencies" % "0.2.16")
 addSbtPlugin("org.scalameta"    % "sbt-scalafmt"              % "2.4.2")
-addSbtPlugin("org.scalameta"    % "sbt-mdoc"                  % "2.2.14")
+addSbtPlugin("org.scalameta"    % "sbt-mdoc"                  % "2.6.1")
 addSbtPlugin("dev.zio"          % "zio-sbt-website"           % "0.3.9")