message: use Buffer.from instead of new Buffer
  (node:70005) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.
diff: @@ -204,7 +204,7 @@ export default class KintoneApiClient {
  username: string,
  password: string
  ): string {
  - const buffer = new Buffer(username + ":" + password);
  + const buffer = Buffer.from(username + ":" + password);
  return buffer.toString("base64");
  }
  