refactor: add missing deprecation

Author: ugur-vaadin

vaadin-split-layout-flow-parent/vaadin-split-layout-flow/src/main/java/com/vaadin/flow/component/splitlayout/SplitLayoutVariant.java

@@ -21,6 +21,10 @@
  * Set of theme variants applicable for {@code vaadin-split-layout} component.
  */
 public enum SplitLayoutVariant implements ThemeVariant {
+    /**
+     * @deprecated Use {@link #SMALL} instead.
+     */
+    @Deprecated(since = "25.0", forRemoval = true)
     LUMO_SMALL("small"), LUMO_MINIMAL("minimal"), SMALL("small");
 
     private final String variant;