Replace success button in the add linked issue

What does this MR do and why?

To replace the success variant button with confirm variant.

This button appears when adding a "Linked issue" on the issue page.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.