Merge pull request #1415 from scala-steward/update/scala-library-2.12.21

Update scala-library from 2.12.20 to 2.12.21

Author: mergify[bot]

project/BuildSettings.scala

@@ -13,7 +13,7 @@ object BuildSettings {
 
   private def isScala3(scalaVersion: String): Boolean = CrossVersion.partialVersion(scalaVersion).exists(_._1 == 3)
 
-  private val scala212 = "2.12.20"
+  private val scala212 = "2.12.21"
   private val scala213 = "2.13.18"
   private val scala3 = "3.3.7"