Update scala-compiler, scala-library, ... to 2.12.21 (#1590)

Co-authored-by: zio-scala-steward[bot] <145262613+zio-scala-steward[bot]@users.noreply.github.com>
Co-authored-by: Jules Ivanic <jules.ivanic@gmail.com>

Author: zio-scala-steward[bot]

project/BuildHelper.scala

@@ -11,7 +11,7 @@ import scala.scalanative.build.Mode
 import scala.scalanative.sbtplugin.ScalaNativePlugin.autoImport.*
 
 object BuildHelper {
-  val Scala212: String = "2.12.20"
+  val Scala212: String = "2.12.21"
   val Scala213: String = "2.13.16"
   val Scala3: String   = "3.3.7"