msglist: Follow web in chevron-right icon color in stream recipient header

The old color (added in a56577a) doesn't match the Figma:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=538-20849&t=sbABZjnTMDdEpaTX-0
  #9A9A9A
or the web app:
  --color-message-header-icon-non-interactive,
  which is hsl(0deg 0% 0% / 30%))

Matching the web app is convenient because we can get a dark-theme
variant from the web app. The Figma doesn't yet offer a dark-theme
variant.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -697,7 +697,7 @@ class StreamMessageRecipientHeader extends StatelessWidget {
               // Icon is 16px wide here so horizontal padding is 1px.
               padding: const EdgeInsets.symmetric(horizontal: 1),
               child: Icon(size: 16,
-                color: Colors.black.withOpacity(0.6),
+                color: Colors.black.withOpacity(0.3),
                 ZulipIcons.chevron_right)),
           ]));
     }