chore(sticky-header): port refactor to v2 (carbon-design-system#11773)

### Description
Ports changes made to feat/masthead-v2-dev branch in carbon-design-system#11734 to v2 branch.

In order to adequately test the new Sticky Header Sandbox story, I had to clean up the Dotcom Shell stories. The `mock` secret menu item works again now, and I've reduced a good amount of bloat in the stories' markup. While I was at it, I noticed I never updated Dotcom Shell's `navLinks` prop for L0 items to Masthead's new `l0Data` prop, so I added support for that as well.

### Testing
The code changes here are mostly identical to the aforementioned PR, with the only divergences being changes necessary for v2. Please verify each of the Dotcom Shell stories works as expected, taking particular care to test the sticky elements behaviors on both desktop and mobile.

packages/styles/scss/components/masthead/_masthead-l1.scss

@@ -141,10 +141,27 @@ $search-transition-timing: 95ms;
     }
 
     .#{$prefix}--masthead__l1-dropdown {
+      position: absolute;
+      background-color: $background;
+      color: $text-secondary;
+      inset-block-start: 100%;
+      inset-inline: 0;
+
+      &.is-open {
+        box-shadow: 0 2px 6px 0 $shadow;
+      }
+
       &:not(.is-open) {
         display: none;
       }
 
+      &:last-child {
+        > a,
+        > button {
+          border-block-end: initial;
+        }
+      }
+
       // Height of viewport, minus the L0/L1 combo, minus additional space to match L0 megapanels
       max-block-size: calc(100vh - 98px - #{$spacing-10} - #{$spacing-09});
       overflow-y: auto;
@@ -343,6 +360,7 @@ $search-transition-timing: 95ms;
         @include type-style(body-short-01);
 
         background-color: $background-brand;
+        border-block-end: initial;
         color: $layer-02;
 
         svg {