Update scala-library to 2.12.20 (#4243)

Author: scala-steward

build.mill

@@ -53,7 +53,7 @@ object Deps {
   val scalaVersion = "2.13.15"
   val scala2Version = "2.13.15"
   // The Scala 2.12.x version to use for some workers
-  val workerScalaVersion212 = "2.12.19"
+  val workerScalaVersion212 = "2.12.20"
 
   val testScala213Version = "2.13.15"
   // Scala Native 4.2 will not get releases for new Scala version