web/changelogs: update cobalt 10 banner

Author: wukko

web/changelogs/10.0.md

@@ -2,8 +2,8 @@
 title: "cobalt, reborn"
 date: "9 Sept, 2024"
 banner:
-    file: "cobalt10.png"
-    alt: "image of meowbalt smiling and loafing in front of a wall of text."
+    file: "cobalt10.webp"
+    alt: "image of a meowth plush staring into a screen with cobalt 10 ui shown."
 ---
 
 everything is new! this update marks the start of the latest chapter for cobalt. we spent the entire summer working hard to deliver the best experience ever, and we really hope you enjoy the rebirth of cobalt.