Update scala-library to 2.12.15

Author: scala-steward

functional-tests/src/main/scala/com/typesafe/tools/mima/lib/TestCli.scala

@@ -9,7 +9,7 @@ import scala.util.{ Properties => StdLibProps }
 
 object TestCli {
   val scala211 = "2.11.12"
-  val scala212 = "2.12.14"
+  val scala212 = "2.12.15"
   val scala213 = "2.13.6"
   val scala3   = "3.0.1"
   val hostScalaVersion = StdLibProps.scalaPropOrNone("maven.version.number").get