refactor(GlDropdown): Remove the `new` prefix in CSS classes

BREAKING CHANGE: rename following CSS class names
- `gl-new-dropdown` => `gl-dropdown`
- `gl-new-dropdown-inner` => `gl-dropdown-inner`
- `gl-new-dropdown-contents` => `gl-dropdown-contents`
- `gl-new-dropdown-header` => `gl-dropdown-header`
- `gl-new-dropdown-header-top` => `gl-dropdown-header-top`
- `gl-new-dropdown-footer` => `gl-dropdown-footer`
- `gl-new-dropdown-item` => `gl-dropdown-item`
- `gl-new-dropdown-item-check-icon` => `gl-dropdown-item-check-icon`
- `gl-new-dropdown-item-icon` => `gl-dropdown-item-icon`
- `gl-new-dropdown-item-text-primary` => `gl-dropdown-item-text-primary`
- `gl-new-dropdown-item-text-secondary`
=> `gl-dropdown-item-text-secondary`
- `gl-new-dropdown-item-text-wrapper` => `gl-dropdown-item-text-wrapper`
- `gl-new-dropdown-divider` => `gl-dropdown-divider`
- `gl-new-dropdown-section-header` => `gl-dropdown-section-header`
- `gl-new-dropdown-text` => `gl-dropdown-text`
- `gl-new-dropdown-button-text` => `gl-dropdown-button-text`