fix(material/card): subtitle text token should be on-surface-variant (angular#31261)

Co-authored-by: Andrew Seguin <andrewseguin@google.com>

Author: andrewseguin

src/material/card/_m2-card.scss

@@ -24,7 +24,7 @@
     card-outlined-container-color: map.get($system, surface),
     card-outlined-container-elevation: elevation.get-box-shadow(0),
     card-outlined-outline-color: inspection.get-theme-color($theme, system, outline),
-    card-subtitle-text-color: map.get($system, surface-variant),
+    card-subtitle-text-color: map.get($system, on-surface-variant),
     card-filled-container-color: map.get($system, surface),
     card-filled-container-elevation: elevation.get-box-shadow(0)
   );