8274879: Replace uses of StringBuffer with StringBuilder within java.base classes

Reviewed-by: naoto

src/java.base/share/classes/java/text/RBTableBuilder.java

@@ -401,7 +401,7 @@ final class RBTableBuilder {
         // can work right
         if (fwd && groupChars.length() > 1) {
             addContractFlags(groupChars);
-            addContractOrder(new StringBuffer(groupChars).reverse().toString(),
+            addContractOrder(new StringBuilder(groupChars).reverse().toString(),
                              anOrder, false);
         }
     }