fix(masthead): fix breakpoint (#4046)

### Description

Found that the latest breakpoint introduced for React masthead (#4021) does not fit well with non-megamenu environment (like what our Web Components codebase is as of today).

Reverted the breakpoint only for Web Components styles, to fix that problem.

### Changelog

**Changed**

- Change the breakpoint of narrow mode masthead back to `lg`.

packages/web-components/src/components/masthead/masthead.scss

@@ -112,6 +112,14 @@
 :host(#{$dds-prefix}-top-nav) {
   @extend :host(#{$prefix}-header-nav);
 
+  @include carbon--breakpoint-up(800px) {
+    display: none;
+  }
+
+  @include carbon--breakpoint('lg') {
+    display: block;
+  }
+
   .#{$prefix}-ce--header__divider {
     background-color: $ui-03;
   }