message: doc(UMDExample): Update itk.js to 12.4.0
diff: <title>itk.js UMD Example</title>
  <meta charset="UTF-8" />
  <link rel="stylesheet" href="styles.css">
  - <script src="https://unpkg.com/itk@12.2.0/umd/itk.js"></script>
  + <script src="https://unpkg.com/itk@12.4.0/umd/itk.js"></script>
  </head>
  <body>
  