Switch success to primary variant in split_button

What does this MR do and why?

Relates to #428004 (closed)

Switch "success" to "primary" variant in split_button according to Pajamas Design System guidelines.

Changelog: fixed

EE: true

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Prerequisites

1. You need an EE license
2. You need to have runners enabled (See $2408961 for setting up a runner)
3. Import https://gitlab.com/gitlab-examples/security/security-reports
4. Run pipeline on master

Validate

Vulnerability details

1. Go to vulnerability report in the security-reports project
2. Filter on the container scanning tool
3. Select a vulnerability to go to its details page
4. Validate that the dropdown button in the top-right corner is now blue – variant "confirm"

Finding modal

1. Go to the latest pipeline's security tab
2. Filter on container scanning tool
3. Select a finding
4. Validate that the dropdown button in the top-bottom corner of the modal is now blue – variant "confirm"

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.