Set defaultSourceVersion to 3.7

Author: WojciechMazur

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

@@ -31,7 +31,7 @@ enum SourceVersion:
   def isAtMost(v: SourceVersion) = stable.ordinal <= v.ordinal
 
 object SourceVersion extends Property.Key[SourceVersion]:
-  def defaultSourceVersion = `3.6`
+  def defaultSourceVersion = `3.7`
 
   /** language versions that may appear in a language import, are deprecated, but not removed from the standard library. */
   val illegalSourceVersionNames = List("3.1-migration", "never").map(_.toTermName)