Update compare branches button to btn-confirm

MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56791

app/views/projects/merge_requests/creations/_new_compare.html.haml

@@ -64,4 +64,4 @@
 
   - if @merge_request.errors.any?
     = form_errors(@merge_request)
-  = f.submit 'Compare branches and continue', class: "gl-button btn btn-success mr-compare-btn"
+  = f.submit 'Compare branches and continue', class: "gl-button btn btn-confirm mr-compare-btn"

changelogs/unreleased/273283-update-compare-page-button.yml

+---
+title: Update compare branches button to btn-confirm
+merge_request: 56791
+author:
+type: changed