JS PIR: replace deprecated toLowerCase with lowercase

compiler/ir/ir.tree.persistent/generator/src/org/jetbrains/kotlin/ir/persistentIrGenerator/PersistentIrGenerator.kt

@@ -274,7 +274,7 @@ internal object PersistentIrGenerator {
 
         protoFields += fields.mapNotNull { f ->
             f.proto?.protoPrefix?.let { p ->
-                val modifier = f.proto.fieldKind.toString().toLowerCase()
+                val modifier = f.proto.fieldKind.toString().lowercase()
                 "$modifier $p ${f.name}"
             }
         }
@@ -708,4 +708,4 @@ internal object PersistentIrGenerator {
             )()
         })
     }
-}
+}