Change variant to confirm for add variable button in modal

What does this MR do and why?

Updates the variant to confirm to match Pajamas guidelines

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Open a project
2. Navigate to /-/settings/ci_cd
3. Add a variable to open the modal
4. Enter details to enable the button

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.