Replace useless @serialversionuid error by a warning

akka has this annotations on some traits and cannot remove it without
breaking binary-compatibility on 2.12, they worked around this error
by using a compiler plugin, but we can just downgrade the error to
warning so they don't have to keep doing something that horrifying.

Author: smarter

compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala

@@ -206,7 +206,7 @@ trait BCodeSkelBuilder extends BCodeHelpers {
       val optSerial: Option[Long] =
         claszSymbol.getAnnotation(defn.SerialVersionUIDAnnot).flatMap { annot =>
           if (claszSymbol.is(Trait)) {
-            report.error("@SerialVersionUID does nothing on a trait", annot.tree.sourcePos)
+            report.warning("@SerialVersionUID does nothing on a trait", annot.tree.sourcePos)
             None
           } else {
             val vuid = annot.argumentConstant(0).map(_.longValue)