Update scala-compiler, scala-library to 2.12.21

Author: scala-steward-bot

build.sbt

@@ -8,7 +8,7 @@ import com.typesafe.tools.mima.core._
 // Scalate project guarantees bin-compatibities for only core, util
 // -----------------------------------------------------------------------------------
 
-def Scala212 = "2.12.20"
+def Scala212 = "2.12.21"
 def Scala213 = "2.13.18"
 def Scala3 = "3.3.7"