Update scala-library, scala-reflect to 2.12.21

Author: scala-steward

build.sbt

@@ -7,7 +7,7 @@ addCommandAlias("full-test", ";+clean;+Test/clean;+test")
 
 Global / onChangedBuildSource := ReloadOnSourceChanges
 
-val Scala212 = "2.12.19"
+val Scala212 = "2.12.21"
 val Scala213 = "2.13.13"
 val Scala3 = "3.3.1"