fix(dropdown): remove excess border of first dropdown element (#11400)

### Related Ticket(s)

#11271 

Closes #11271 

### Description

Just querying first element of dropdown and removing its `border-block-start-width`

### Changelog

**Changed**

- updated `dropdown.scss` with 5 new lines

packages/carbon-web-components/src/components/dropdown/dropdown.scss

@@ -1,5 +1,5 @@
 //
-// Copyright IBM Corp. 2019, 2023
+// Copyright IBM Corp. 2019, 2024
 //
 // This source code is licensed under the Apache-2.0 license found in the
 // LICENSE file in the root directory of this source tree.
@@ -101,6 +101,12 @@ $css--plex: true !default;
   }
 }
 
+:host(#{$prefix}-dropdown-item:first-child) {
+  .#{$prefix}--list-box__menu-item__option {
+    border-block-start-width: 0;
+  }
+}
+
 :host(#{$prefix}-dropdown-item:hover) {
   background-color: $layer-hover;
 }