Introduce the SourceVersions 3.4 and 3.4-migration; make 3.4 the default.

Author: sjrd

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

@@ -9,6 +9,7 @@ enum SourceVersion:
   case `3.0-migration`, `3.0`, `3.1` // Note: do not add `3.1-migration` here, 3.1 is the same language as 3.0.
   case `3.2-migration`, `3.2`
   case `3.3-migration`, `3.3`
+  case `3.4-migration`, `3.4`
   case `future-migration`, `future`
 
   val isMigrating: Boolean = toString.endsWith("-migration")
@@ -21,7 +22,7 @@ enum SourceVersion:
   def isAtMost(v: SourceVersion) = stable.ordinal <= v.ordinal
 
 object SourceVersion extends Property.Key[SourceVersion]:
-  def defaultSourceVersion = `3.3`
+  def defaultSourceVersion = `3.4`
 
   /** language versions that may appear in a language import, are deprecated, but not removed from the standard library. */
   val illegalSourceVersionNames = List("3.1-migration").map(_.toTermName)