[fenix] For https://github.com/mozilla-mobile/fenix/issues/26283: Use message buttonLabel in MessageCard.

app/src/main/java/org/mozilla/fenix/compose/MessageCard.kt

@@ -124,11 +124,12 @@ fun MessageCard(
                 }
             }
 
-            if (!message.data.buttonLabel.isNullOrBlank()) {
+            val buttonLabel = message.data.buttonLabel
+            if (!buttonLabel.isNullOrBlank()) {
                 Spacer(modifier = Modifier.height(16.dp))
 
                 PrimaryButton(
-                    text = stringResource(R.string.preferences_set_as_default_browser),
+                    text = buttonLabel,
                     onClick = onClick
                 )
             }