fix(GlDropdownItem): Fix dropdown item disabled state styling

What does this MR do?

This MR fixes GlDropdownItem not looking disabled when the disabled attribute is added to it.

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Closes #2092 (closed)

src/components/base/dropdown/dropdown_item.scss

@@ -69,6 +69,14 @@
       .gl-new-dropdown-item-check-icon {
         @include gl-reset-color;
       }
+
+      .gl-new-dropdown-item-text-secondary {
+        @include gl-reset-color;
+      }
+
+      .gl-avatar {
+        @include gl-opacity-7;
+      }
     }
 
     &:not(.disable-hover):hover,