Update social media links and styles in slides; replace Twitter with BlueSky and update Font Awesome version

slides/themes/custom-default.css

@@ -21,10 +21,11 @@ img[alt~="center"] {
 .fa-mastodon { color: purple; }
 .fa-linkedin { color: blue; }
 .fa-window-maximize { color: skyblue; }
+.fa-bluesky { color: skyblue; }
 
 svg[id^="mermaid-"] { 
   max-width: 960px; 
   max-height: 600px; 
 }
 
-@import 'https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.3.0/css/all.min.css'
+@import 'https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.7.1/css/all.min.css';