content-message-row: Use 'caption' CSS class

Instead of 'heading'. The original mockup [1] seems to have this label
much more subtle, and definitely not bold.

[1] https://gitlab.gnome.org/Teams/Design/app-mockups/-/raw/master/fractal/main-window.png

data/resources/ui/content-message-row.ui

@@ -32,7 +32,7 @@
                 <child type="end">
                   <object class="GtkLabel" id="timestamp">
                     <style>
-                      <class name="heading"/>
+                      <class name="caption"/>
                       <class name="timestamp"/>
                       <class name="dim-label"/>
                     </style>