use menu icon less variable (#19553)

components/menu/style/index.less

@@ -160,7 +160,7 @@
     .@{iconfont-css-prefix} {
       min-width: 14px;
       margin-right: 10px;
-      font-size: @font-size-base;
+      font-size: @menu-icon-size;
       transition: font-size 0.15s @ease-out, margin 0.3s @ease-in-out;
       + span {
         opacity: 1;
@@ -411,14 +411,14 @@
       > .@{menu-prefix-cls}-submenu-title,
     > .@{menu-prefix-cls}-submenu > .@{menu-prefix-cls}-submenu-title {
       left: 0;
-      padding: 0 (@menu-collapsed-width - 16px) / 2 !important;
+      padding: 0 (@menu-collapsed-width - @menu-icon-size-lg) / 2 !important;
       text-overflow: clip;
       .@{menu-prefix-cls}-submenu-arrow {
         display: none;
       }
       .@{iconfont-css-prefix} {
         margin: 0;
-        font-size: 16px;
+        font-size: @menu-icon-size-lg;
         line-height: @menu-item-height;
         + span {
           display: inline-block;