Change button variant to confirm in the user list of the feature flag

What does this MR do and why?

Fix the button variant success -> confirm

Screenshots or screen recordings

As-is	To-be

How to set up and validate locally

1. Go to the project on the GDK
2. Go to Deployments > Feature flags
3. Hit the User list button on the right top (without bordered one)
4. Create a user list and the screen will appear.

I used http://127.0.0.1:3000/flightjs/Flight/-/feature_flags_user_lists/new

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.