Update scala-library to 2.12.21

Author: typelevel-steward[bot]

build.sbt

@@ -12,7 +12,7 @@ val scalacheckEffect = "2.1.0"
 val refinedVersion = "0.11.3"
 val nakedFSVersion = "0.1.0"
 
-val Scala212 = "2.12.20"
+val Scala212 = "2.12.21"
 val Scala213 = "2.13.18"
 
 ThisBuild / tlBaseVersion := "0.17"