Use confirm variant for 'add' button in user lists modal

What does this MR do and why?

This MR changes the button variant of the Add button in the user lists "Add user" modal to confirm, so it's easier to identify for the user (intuitively!) which button adds the entered users.

Screenshots or screen recordings

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How to set up and validate locally

1. Go to a project
2. Go to Deployments > Feature Flags
3. Click on View user lists
4. Create or edit an existing user list
5. Click on Add User
6. See the modal

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.