msglist: Follow web in recipient-header-date color

The old color, which we started using for stream headers in
e97e6fd and DM headers in 81f7038, doesn't match the Figma:
  https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=538-20849&t=ubXJTWC5kuz5awbN-0
  #666666
or the web app:
  --color-date,
  which is hsl(0deg 0% 15% / 75%)

I wonder if some confusion about contrastingColor (see a recent
commit where we removed a variable with that name) might have led to
the old color being used for stream headers, and perhaps the
DM-header change was just done to align with stream headers.

Anyway, might as well use the web app's color, which we were doing
before e97e6fd. We can get a dark-theme variant from the web app
too, which is nice. The Figma doesn't yet offer a dark-theme
variant.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -519,7 +519,7 @@ class DateSeparator extends StatelessWidget {
 
   final Message message;
 
-  // This color matches recipient headers.  TODO(design) is that what we want?
+  // TODO(design) do we want this color to match recipient headers?
   static final _textColor = Colors.black.withOpacity(0.4);
 
   @override
@@ -801,7 +801,7 @@ class RecipientHeaderDate extends StatelessWidget {
     return Padding(
       padding: const EdgeInsets.fromLTRB(10, 0, 16, 0),
       child: DateText(
-        color: Colors.black.withOpacity(0.4),
+        color: const HSLColor.fromAHSL(0.75, 0, 0, 0.15).toColor(),
         fontSize: 16,
         // In Figma this has a line-height of 19, but using 18
         // here to match the stream/topic text widgets helps