Update Scala to 2.13.8

Author: alexarchambault

Diff for: build.sc

@@ -11,7 +11,7 @@ import mill.scalalib._
 def scalafmtVersion = "3.4.3"
 
 trait ScalafmtNativeImage extends ScalaModule with NativeImage {
-  def scalaVersion = "2.13.6"
+  def scalaVersion = "2.13.8"
 
   def nativeImageClassPath = T{
     runClasspath()