re-deprecate @screen-xs-min per #12913

By definition, XS screens have no minimum size, only a maximum size.
(Symmetrically, LG screens have no maximum size, hence why there's no @screen-lg-max.)

less/variables.less

@@ -262,6 +262,7 @@
 
 // Extra small screen / phone
 // Note: Deprecated @screen-xs and @screen-phone as of v3.0.1
+// Note: Deprecated @screen-xs-min as of v3.2.0
 @screen-xs:                  480px;
 @screen-xs-min:              @screen-xs;
 @screen-phone:               @screen-xs-min;