style: update not available text color in engagements page

- Modify the style of the 'not available' text in the engagements page
- Use onSurface color with 60% opacity for better visibility

Author: fulleni

lib/community_management/view/engagements_page.dart

@@ -212,7 +212,9 @@ class _EngagementsDataSource extends DataTableSource {
                 )
               : Text(
                   l10n.notAvailable,
-                  style: Theme.of(context).textTheme.bodySmall,
+                  style: Theme.of(context).textTheme.bodySmall?.copyWith(
+                        color: Theme.of(context).colorScheme.onSurface.withOpacity(0.6),
+                      ),
                 ),
         ),
         DataCell(