Update the design management empty state button style

Problem to solve

The design management empty state added in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10599 included a primary button to upload designs with blue color. According to the design system, green color is used for actions like this.

Blue indicates a current or active state. It communicates: management, progress, connectivity, or organization.

Green indicates success. It communicates: save, create, add, available, done, approved, or resolved.

See https://design.gitlab.com/product-foundations/colors.

Proposal

Let's update the button color to green.