update 2.12 version

Author: coltfred

build.sbt

@@ -2,7 +2,7 @@ lazy val commonSettings = Seq(
   organization := "com.whisk",
   version := "0.13.0",
   scalaVersion := "2.13.16",
-  crossScalaVersions := Seq("2.13.16", "2.12.15", "3.0.2"),
+  crossScalaVersions := Seq("2.13.16", "2.12.20", "3.0.2"),
   scalacOptions ++= Seq("-feature", "-deprecation"),
   Test / fork := true,
   licenses += ("MIT", url("http://opensource.org/licenses/MIT")),