refactor(new_dropdown): Use splitClass property to style splitButton

What does this MR do?

This MR addresses a technical debt introduced before upgrading to BootstrapVue 2.13.0. Before BV2.13.0, the BDropdown component didn’t provide a property to style the split button. To circumvent this limitation, we added the styling classes by modifying BBootstrap internals when the component was mounted. This MR replaces this approach with the splitClass property.

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
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.