Update unprotect branch button variant and size

Changelog: changed

app/views/projects/protected_branches/shared/_protected_branch.html.haml

@@ -20,4 +20,4 @@
 
   - if can_admin_project
     %td
-      = link_to s_('ProtectedBranch|Unprotect'), [@project, protected_branch, { update_section: 'js-protected-branches-settings' }], disabled: local_assigns[:disabled], aria: { label: s_('ProtectedBranch|Unprotect branch') }, data: { confirm: s_('ProtectedBranch|Branch will be writable for developers. Are you sure?'), confirm_btn_variant: 'danger' }, method: :delete, class: "btn gl-button btn-warning"
+      = link_to s_('ProtectedBranch|Unprotect'), [@project, protected_branch, { update_section: 'js-protected-branches-settings' }], disabled: local_assigns[:disabled], aria: { label: s_('ProtectedBranch|Unprotect branch') }, data: { confirm: s_('ProtectedBranch|Branch will be writable for developers. Are you sure?'), confirm_btn_variant: 'danger' }, method: :delete, class: "btn gl-button btn-danger btn-sm"