Add btn-default for mirror update button

changelogs/unreleased/btn-default-mirror-update.yml

+---
+title: Add btn-default for mirror update button
+merge_request: 57978
+author: Yogi (@yo)
+type: changed

ee/app/views/shared/_mirror_update_button.html.haml

 - if @project.mirror? && can?(current_user, :push_code, @project)
   .gl-mb-3
     - if @project.import_state.mirror_update_due?
-      %span.btn.gl-button.disabled
+      %span.btn.gl-button.btn-default.disabled
         = sprite_icon('retry', css_class: 'spin gl-mr-3')
         = ('Update Scheduled…')
     - elsif @project.import_state.updating_mirror?
-      %span.btn.gl-button.disabled
+      %span.btn.gl-button.btn-default.disabled
         = sprite_icon('retry', css_class: 'spin gl-mr-3')
         = ('Updating…')
     - elsif can?(current_user, :admin_project, @project)
-      = link_to update_now_project_mirror_path(@project), method: :post, class: 'btn' do
+      = link_to update_now_project_mirror_path(@project), method: :post, class: 'gl-button btn btn-default' do
         = sprite_icon('retry', css_class: 'gl-mr-3')
         = ('Update Now')
     - else
-      %span.btn.gl-button.disabled
+      %span.btn.gl-button.btn-default.disabled
         = sprite_icon('retry', css_class: 'gl-mr-3')
         = ('Update Now')
     - if @project.mirror_last_update_succeeded?