Adjust main encryption toast to reference Secure Backup

This adjusts the main toast to focus on Secure Backup as suggested in designs.

Part of https://github.com/vector-im/element-web/issues/13895

res/css/structures/_ToastContainer.scss

@@ -80,6 +80,11 @@ limitations under the License.
                 }
             }
 
+            &.mx_Toast_icon_secure_backup::after {
+                mask-image: url('$(res)/img/feather-customised/secure-backup.svg');
+                background-color: $primary-fg-color;
+            }
+
             .mx_Toast_title, .mx_Toast_body {
                 grid-column: 2;
             }