Update Scala 2.12 to 2.12.21

Author: lefou

mill-build/src/millbuild/Deps.scala

@@ -20,7 +20,7 @@ object Deps {
   val scalaVersionJava11 = "3.7.4"
   val scala2Version = "2.13.18"
   // The Scala 2.12.x version to use for some workers
-  val workerScalaVersion212 = "2.12.20"
+  val workerScalaVersion212 = "2.12.21"
   val sbtScalaVersion212 = workerScalaVersion212
 
   val testScala213Version = "2.13.18"