Migrate to tertiary buttons in DropdownContentsCreateView

What does this MR do and why?

Migrates both link buttons to tertiary buttons in the DropdownContentsCreateView component.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Ensure your GDK has an GitLab Ultimate license.
2. Apply the following patch to short-circuit a condition to get the right component to render:

   diff --git a/app/assets/javascripts/vue_shared/components/sidebar/labels_select_vue/dropdown_contents.vue b/app/assets/javascripts/vue_shared/components/sidebar/labels_select_vue/dropdown_contents.vue
   index 399db978b60..7b99479d871 100644
   --- a/app/assets/javascripts/vue_shared/components/sidebar/labels_select_vue/dropdown_contents.vue
   +++ b/app/assets/javascripts/vue_shared/components/sidebar/labels_select_vue/dropdown_contents.vue
   @@ -20,7 +20,7 @@ export default {
        ...mapState(['showDropdownContentsCreateView']),
        ...mapGetters(['isDropdownVariantSidebar']),
        dropdownContentsView() {
   -      if (this.showDropdownContentsCreateView) {
   +      if (true) {
            return 'dropdown-contents-create-view';
          }
          return 'dropdown-contents-labels-view';

3. Navigate to a group's epics list.
4. Click on Edit epics.
5. Open the Labels dropdown in the sidebar.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #241572 (closed)