Add confirm variant to primary button

What does this MR do and why?

Add confirm variant to primary button

The modal, which has a form and therefore a custom submit button, now displays the confirm button in the confirm style of GitLab UI.

Changelog: changed EE: true

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to any group
2. Click on "Settings" > "Service Accounts"
3. Click on "Add service account"

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