Update custom_render.html

[markvantilburg]

Fixes mixed content error logging from the demo website over at jqueryui.com

Mixed Content: The page at '<URL>' was loaded over HTTPS, but requested an insecure element '<URL>'. This request was automatically upgraded to HTTPS, For more information see <URL> VM25 jquery-3.6.0.js:6591 Mixed Content: The page at 'https://jqueryui.com/resources/demos/selectmenu/custom_render.html' was loaded over HTTPS, but requested an insecure element 'http://www.gravatar.com/avatar/b3e04a4…?d=monsterid&r=g&s=16'. This request was automatically upgraded to HTTPS, For more information see https://blog.chromium.org/2019/10/no-more-mixed-messages-about-https.html 

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: markvantilburg / name: mark van tilburg (25025a3, 8d9de29, d99cd99, ee30489, b4348d3, 43bc29a, 2ce9492, 64a198b, e771979, 8b34f0f, 6d4e6dc, d9a9964, 01fd13b, 7f1657d)

[mgol]

Thanks for the PR. I think I'd like to update all HTTP URLs to HTTPS in one go, at least the ones used in demos. Would you like to try that?

[markvantilburg]

see also: #2183

[markvantilburg]

updated all http:// links in the demos also checked if these pages are available over https

[mgol]

Thank you! Especially for testing URL availability on HTTPS. 🙂

[mgol]

I also fixed #2183 and then I updated remaining HTTP URLs to HTTPS in #2244.