Bump Scala 3 Next RC to 3.8.0-RC3 (#3991)

Author: Gedochao

project/deps/package.mill.scala

@@ -20,7 +20,7 @@ object Scala {
   def scala3Next          = s"$scala3NextPrefix.4" // the newest/next version of Scala
   def scala3NextAnnounced =
     s"$scala3NextPrefix.3" // the newest/next version of Scala that's been announced
-  def scala3NextRc        = "3.8.0-RC2"            // the latest RC version of Scala Next
+  def scala3NextRc        = "3.8.0-RC3"            // the latest RC version of Scala Next
   def scala3NextRcAnnounced = "3.7.4-RC3" // the latest announced RC version of Scala Next
 
   // The Scala version used to build the CLI itself.