Set -source default to 3.8

Author: WojciechMazur

compiler/src/dotty/tools/dotc/config/SourceVersion.scala

@@ -49,7 +49,7 @@ enum SourceVersion:
 object SourceVersion extends Property.Key[SourceVersion]:
 
   /* The default source version used by the built compiler */
-  val defaultSourceVersion = `3.7`
+  val defaultSourceVersion = `3.8`
 
   /* Illegal source versions that may not appear in the settings `-source:<...>` */
   val illegalInSettings = List(`2.13`, `3.1-migration`, `never`)