[bug]: <rh-menu-dropdown> missing css var fallbacks #3179

Tracked by #3119

Missing fallbacks

• red-hat-design-system/elements/rh-menu-dropdown/rh-menu-dropdown.css

  Line 35 in 6cfaf01

  --_color: var(--rh-color-text-primary);

Part of the effort to ensure all component CSS custom property references include correct fallback values.