Make right part of split dropdown square-shaped in GitLab

Specs

(https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/buttons-spec-previews/)

<gl-new-dropdown split> in gitlab-ui

(https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-new-dropdown--default)

states:

Note: This component is still being developed. Please use [<gl-dropdown>] instead.

<gl-dropdown split> in gitlab-ui

(https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-dropdown--default)

<gl-dropdown split> in GitLab