Update mdoc, sbt-mdoc to 2.8.0 (#606)

Co-authored-by: Rui Gonçalves <ruippeixotog@gmail.com>

Author: scala-steward

project/plugins.sbt

@@ -1,7 +1,7 @@
 addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.14.4")
 addSbtPlugin("com.github.sbt" % "sbt-release" % "1.4.0")
 addSbtPlugin("com.github.sbt" % "sbt-pgp" % "2.3.1")
-addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.7.2")
+addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.8.0")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.5")
 addSbtPlugin("org.scoverage" % "sbt-coveralls" % "1.3.15")
 addSbtPlugin("org.scoverage" % "sbt-scoverage" % "2.4.0")