Fix accidentally enabled warning about open classes

Author: odersky

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1613,7 +1613,7 @@ class Namer { typer: Typer =>
             else if pclazz.isEffectivelySealed && pclazz.associatedFile != cls.associatedFile then
               if pclazz.is(Sealed) && !pclazz.is(JavaDefined) then
                 report.error(UnableToExtendSealedClass(pclazz), cls.srcPos)
-              else if sourceVersion.isAtLeast(`3.6`) then
+              else if sourceVersion.isAtLeast(future) then
                 checkFeature(nme.adhocExtensions,
                   i"Unless $pclazz is declared 'open', its extension in a separate file",
                   cls.topLevelClass,