Migrate checkbox to be GitLab UI component

app/views/admin/application_settings/_repository_mirrors_form.html.haml

-= form_for @application_setting, url: repository_admin_application_settings_path(anchor: 'js-mirror-settings') do |f|
+= gitlab_ui_form_for @application_setting, url: repository_admin_application_settings_path(anchor: 'js-mirror-settings') do |f|
   = form_errors(@application_setting)
 
   %fieldset
     .form-group
       = f.label :mirror_available, _('Repository mirroring configuration'), class: 'label-bold'
-      .form-check
-        = f.check_box :mirror_available, class: 'form-check-input'
-        = f.label :mirror_available, class: 'form-check-label' do
-          = _('Allow project maintainers to configure repository mirroring')
-      %span.form-text.text-muted
-        = _('If disabled, only administrators can configure repository mirroring.')
+      = f.gitlab_ui_checkbox_component :mirror_available, _('Allow project maintainers to configure repository mirroring'),
+      help_text: _('If disabled, only administrators can configure repository mirroring.')
 
     = render_if_exists 'admin/application_settings/mirror_settings', form: f