Merge pull request #191 from outr/update/scala-library-2.12.21

Update scala-library to 2.12.21

Author: darkfrog26

build.sbt

@@ -3,7 +3,7 @@ import sbtcrossproject.CrossPlugin.autoImport.{crossProject, CrossType}
 // Scala versions
 val scala213 = "2.13.17"
 
-val scala212 = "2.12.20"
+val scala212 = "2.12.21"
 
 val scala3 = List("3.3.7")