msglist: Update "Mark all messages as read" button icon

Icon taken from the Figma design:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=3512-7&t=FEydAZe5DNKVlKnV-0

Fixes: zulip#1357

Author: lakshya1goel

assets/icons/ZulipIcons.ttf

@@ -102,44 +102,47 @@ abstract final class ZulipIcons {
   /// The Zulip custom icon "menu".
   static const IconData menu = IconData(0xf11a, fontFamily: "Zulip Icons");
 
+  /// The Zulip custom icon "message_checked".
+  static const IconData message_checked = IconData(0xf11b, fontFamily: "Zulip Icons");
+
   /// The Zulip custom icon "message_feed".
-  static const IconData message_feed = IconData(0xf11b, fontFamily: "Zulip Icons");
+  static const IconData message_feed = IconData(0xf11c, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "mute".
-  static const IconData mute = IconData(0xf11c, fontFamily: "Zulip Icons");
+  static const IconData mute = IconData(0xf11d, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "read_receipts".
-  static const IconData read_receipts = IconData(0xf11d, fontFamily: "Zulip Icons");
+  static const IconData read_receipts = IconData(0xf11e, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "send".
-  static const IconData send = IconData(0xf11e, fontFamily: "Zulip Icons");
+  static const IconData send = IconData(0xf11f, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "share".
-  static const IconData share = IconData(0xf11f, fontFamily: "Zulip Icons");
+  static const IconData share = IconData(0xf120, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "share_ios".
-  static const IconData share_ios = IconData(0xf120, fontFamily: "Zulip Icons");
+  static const IconData share_ios = IconData(0xf121, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "smile".
-  static const IconData smile = IconData(0xf121, fontFamily: "Zulip Icons");
+  static const IconData smile = IconData(0xf122, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "star".
-  static const IconData star = IconData(0xf122, fontFamily: "Zulip Icons");
+  static const IconData star = IconData(0xf123, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "star_filled".
-  static const IconData star_filled = IconData(0xf123, fontFamily: "Zulip Icons");
+  static const IconData star_filled = IconData(0xf124, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "three_person".
-  static const IconData three_person = IconData(0xf124, fontFamily: "Zulip Icons");
+  static const IconData three_person = IconData(0xf125, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "topic".
-  static const IconData topic = IconData(0xf125, fontFamily: "Zulip Icons");
+  static const IconData topic = IconData(0xf126, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "unmute".
-  static const IconData unmute = IconData(0xf126, fontFamily: "Zulip Icons");
+  static const IconData unmute = IconData(0xf127, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "user".
-  static const IconData user = IconData(0xf127, fontFamily: "Zulip Icons");
+  static const IconData user = IconData(0xf128, fontFamily: "Zulip Icons");
 
   // END GENERATED ICON DATA
 }

assets/icons/message_checked.svg

@@ -826,7 +826,7 @@ class _MarkAsReadWidgetState extends State<MarkAsReadWidget> {
                 backgroundColor: WidgetStatePropertyAll(messageListTheme.unreadMarker),
               ),
               onPressed: _loading ? null : () => _handlePress(context),
-              icon: const Icon(Icons.playlist_add_check),
+              icon: const Icon(ZulipIcons.message_checked),
               label: Text(zulipLocalizations.markAllAsReadLabel))))));
   }
 }