Update scala-library, scala-reflect to 2.12.20

Author: scala-steward

build.sbt

@@ -17,7 +17,7 @@ import laika.config._
 import sbt.Def._
 import scala.scalanative.build._
 
-val scala212 = "2.12.19"
+val scala212 = "2.12.20"
 val scala213 = "2.13.14"
 val scala3 = "3.3.3"
 val fs2Version = "3.11.0"