Update Font Awesome to v6.5.1 and replace Twitter icon with X icon

[Copilot]

Description

Upgrades Font Awesome from 4.4.0 to 6.5.1 CDN and replaces Twitter bird icon with X icon.

Changes

Font Awesome upgrade:

• Replaced local v4.4.0 files with CDN links (cdnjs.cloudflare.com/ajax/libs/font-awesome/6.5.1/)
• Added v4-shims.min.css for backward compatibility with existing icon classes
• Removed css/font-awesome/ directory (~2,600 lines)
• Fixed 404.html to remove duplicate Font Awesome link (now uses default layout's head.html)

Twitter icon update:

• Changed fa fa-twitter to fa-brands fa-x-twitter in index.html and past.html
• X icon now displays for all conference Twitter handles

- <i class="fa fa-twitter"></i>
+ <i class="fa-brands fa-x-twitter"></i>

Checklist

• Build runs successfully
• No security vulnerabilities detected

Additional context

The v4 shims maintain compatibility with existing icons (calendar, map-marker, bell, video-camera, home) without code changes. CDN delivery improves performance and reduces repository size.

CI Fix: Resolved build failure where 404.html was referencing the removed local Font Awesome files. The page now properly inherits Font Awesome from the default layout, eliminating duplicate stylesheet references.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update Twitter Icon</issue_title>
<issue_description>- [ ] Install the latest version of font-awesome

• Make sure the latest twitter icon is the x icon</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Update Twitter Icon #802

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[ckenst]

@copilot there's a CI failure:

  *  internally linking to /css/font-awesome/css/font-awesome.min.css, which does not exist (line 123)
     <link rel="stylesheet" href="/css/font-awesome/css/font-awesome.min.css">```

Please check the 404.html page for its use of font-awesome.
  

[Copilot]

@copilot there's a CI failure:

...

Fixed in commit 2d782c4. Updated 404.html to use the Font Awesome 6.5.1 CDN links instead of the removed local files. The page now includes both the main CSS and v4-shims for backward compatibility with the home icon.