Restyle add users button

What does this MR do and why?

This MR changes the variant of the Add Users button of user lists to confirm as it's the standard for primary actions.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to a project
2. Go to feature flags
3. Click on View user lists
4. Create a new user list
5. See the Add Users 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.