Cross build against Scala 2.12 because it's need by a sbt-plugin

The project is defined as interplay PlayLibrary, which however dropped
Scala 2.12, because a PlayLibrary is meant to be used for libraries
not for plugins. So we need to explicitly build for 2.12.

.github/workflows/build-test.yml

@@ -26,7 +26,7 @@ jobs:
     uses: playframework/.github/.github/workflows/cmd.yml@v2
     with:
       java: 11, 8
-      scala: 2.13.8
+      scala: 2.12.15, 2.13.8
       cmd: sbt ++$MATRIX_SCALA test
 
   finish:

build.sbt

@@ -1,3 +1,5 @@
+import interplay.ScalaVersions._
+
 // Customise sbt-dynver's behaviour to make it work with tags which aren't v-prefixed
 (ThisBuild / dynverVTagPrefix) := false
 
@@ -10,6 +12,9 @@ Global / onLoad := (Global / onLoad).value.andThen { s =>
 
 lazy val `play-doc` = (project in file("."))
   .enablePlugins(PlayLibrary, SbtTwirl, PlayReleaseBase)
+  .settings(
+    crossScalaVersions := Seq(scala212, scala213)
+  )
 
 libraryDependencies ++= Seq(
   "org.pegdown" % "pegdown"     % "1.6.0",