Update scala-library to 2.12.15

Author: 47erbot

build.sbt

@@ -1,7 +1,7 @@
 ThisBuild / organization       := "org.scala-exercises"
 ThisBuild / githubOrganization := "47degrees"
 ThisBuild / scalaVersion       := "2.13.3"
-ThisBuild / crossScalaVersions := Seq("2.13.3", "2.12.11")
+ThisBuild / crossScalaVersions := Seq("2.13.3", "2.12.15")
 
 publish / skip := true