Migrate buttons in epic tree to confirm variant

What does this MR do and why?

Migrate deprecated success (green) buttons to the confirm (blue) variant for the 'Create issue' and 'Create epic' buttons that allow a user to add a new issue or epic to the tree.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. View an epic (possible in the review app).
2. Under the Epics and Issues tab use the Add dropdown to choose one of the following:
   1. Add a new issue
   2. Add a new epic

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.

Related to #345928 (closed)