cross compile to 2.12 and 2.13 (#65)

Author: wing328

build.sbt

@@ -9,8 +9,8 @@ specifications as part of your build. Other tasks are available as command line
 
 lazy val `sbt-openapi-generator` = (project in file("."))
   .settings(
-    scalaVersion := "2.12.20",
-    //crossScalaVersions := Seq(scalaVersion.value, "2.11.12"),
+    scalaVersion := "2.13.16",
+    crossScalaVersions := Seq(scalaVersion.value, "2.12.20"),
     crossSbtVersions := List("0.13.17", "1.3.10"),
     sbtPlugin := true,