only unidoc for Scala 2.12, #25985

* otherwise the docs built with 2.11 will be published for release,
  since the 2.11 build is last

build.sbt

@@ -53,7 +53,12 @@ lazy val root = Project(
   base = file(".")
 ).aggregate(aggregatedProjects: _*)
  .settings(rootSettings: _*)
- .settings(unidocRootIgnoreProjects := Seq(remoteTests, benchJmh, protobuf, akkaScalaNightly, docs))
+ .settings(unidocRootIgnoreProjects :=
+   (CrossVersion.partialVersion(scalaVersion.value) match {
+     case Some((2, n)) if n == 11 ⇒ aggregatedProjects // ignore all, don't unidoc when scalaVersion is 2.11
+     case _                       ⇒ Seq(remoteTests, benchJmh, protobuf, akkaScalaNightly, docs) 
+   })
+ )
  .settings(
    unmanagedSources in(Compile, headerCreate) := (baseDirectory.value / "project").**("*.scala").get
  )