Activate unchecked match rewrite for 3.1-migration

Author: adpi2

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

@@ -1364,7 +1364,7 @@ class Typer extends Namer
             tree.selector.removeAttachment(desugar.CheckIrrefutable) match {
               case Some(checkMode) =>
                 val isPatDef = checkMode == desugar.MatchCheck.IrrefutablePatDef
-                if (!checkIrrefutable(pat, sel.tpe, isPatDef) && ctx.settings.migration.value)
+                if (!checkIrrefutable(pat, sel.tpe, isPatDef) && sourceVersion == `3.1-migration`)
                   if (isPatDef) patch(Span(pat.span.end), ": @unchecked")
                   else patch(Span(pat.span.start), "case ")
               case _ =>