Update the demo to use non-deprecated APIs.

1) Replace `document.registerElement` with `customElements.define`.
2) Replace `createShadowRoot` with `attachShadow`.
3) Get rid of `document.writeln`, which is unsupported and unneeded.

Up-to-date as of Oct 23, 2022.

index.html

@@ -5,11 +5,6 @@
 <title>HTML5 &lt;x-knob&gt; rotating knob web component</title>
 <meta name="viewport" content="width=device-width, initial-scale=1">
 
-<!--
-Required for browsers that do not yet support the latest technologies.
-http://webcomponents.org/
-https://github.com/webcomponents/webcomponentsjs/releases
--->
 <script type="text/javascript">
 if (!document.registerElement) {
  document.writeln(decodeURI('%3Cscript%20src=%22https://cdnjs.cloudflare.com/ajax/libs/webcomponentsjs/0.7.12/webcomponents.min.js%22%20type=%22text/javascript%22%3E%3C/script%3E'));