Fix Service Templates deprecation notice button

Should include btn-default class to change the background
from transparent to white.

app/views/admin/services/index.html.haml

@@ -11,7 +11,7 @@
       = s_('AdminSettings|Try using the latest version of Integrations instead.')
     .gl-alert-actions
       = link_to _('Go to Integrations'), integrations_admin_application_settings_path, class: 'btn btn-info gl-alert-action gl-button'
-      = link_to _('Learn more'), help_page_path('user/admin_area/settings/project_integration_management'), class: 'btn gl-alert-action btn-secondary gl-button', target: '_blank', rel: 'noopener noreferrer'
+      = link_to _('Learn more'), help_page_path('user/admin_area/settings/project_integration_management'), class: 'btn btn-default gl-alert-action btn-secondary gl-button', target: '_blank', rel: 'noopener noreferrer'
 
 %h3.page-title Service templates
 %p.light= s_('AdminSettings|Service template allows you to set default values for integrations')