Update scala-library to 2.12.21

Author: scala-steward

build.sbt

@@ -4,7 +4,7 @@ import Keys._
 import sbtsoftwaremill.BuildInfo
 
 val commonSettings = Publish.ossPublishSettings ++ Seq(
-  scalaVersion := "2.12.20",
+  scalaVersion := "2.12.21",
   organization := "com.softwaremill.sbt-softwaremill",
   sbtVersion in Global := {
     scalaBinaryVersion.value match {