Include Scala 3 in cross-build (#30392)

* Include Scala 3 in cross-build

sbt cross-building sometimes behaves surprisingly, so this does not work
yet: when switching to 3.0.1-RC1 it still tries to build the modules
that do not support that version yet, even though they are 'excluded'.

This also currently breaks cross-publishing, so we cannot merge this.
Once this works we should add a note to the documentation clarifying
that the Scala 3 artifacts are experimental.

* Fix jackson test dependency

* Don't publish docs for scala3 artifacts for now

* Publish empty doc packages for Scala 3 artifacts

project/Doc.scala

@@ -35,7 +35,14 @@ object Scaladoc extends AutoPlugin {
         // -release caused build failures when generating javadoc:
         Compile / scalacOptions --= Seq("-release", "8"),
         autoAPIMappings := CliOptions.scaladocAutoAPI.get)) ++
-    Seq(Compile / validateDiagrams := true) ++
+    Seq(
+      // Publishing scala3 docs is broken (https://github.com/akka/akka/issues/30788),
+      // for now we just skip it:
+      Compile / doc / sources := (
+          if (scalaVersion.value.startsWith("3.")) Seq()
+          else (Compile / doc / sources).value
+        ),
+      Compile / validateDiagrams := true) ++
     CliOptions.scaladocDiagramsEnabled.ifTrue(Compile / doc := {
       val docs = (Compile / doc).value
       if ((Compile / validateDiagrams).value)