Update scala-library to 2.12.21 (#278)

Author: hnaderi

build.sbt

@@ -15,7 +15,7 @@ ThisBuild / tlSitePublishBranch := Some("main")
 ThisBuild / tlCiReleaseBranches := Seq("main")
 ThisBuild / tlFatalWarnings := false
 
-val Scala212 = "2.12.20"
+val Scala212 = "2.12.21"
 val Scala213 = "2.13.18"
 val Scala3 = "3.3.7"