Add warning to when converting runner to specific

This change adds a simple warning to administrators when restricting a
shared/instance runner as this operation is not reversible.

Changelog: changed

app/views/admin/runners/show.html.haml

@@ -60,7 +60,7 @@
             .float-right
               = form_for project.runner_projects.new, url: admin_namespace_project_runner_projects_path(project.namespace, project), method: :post do |f|
                 = f.hidden_field :runner_id, value: @runner.id
-                = f.submit _('Enable'), class: 'gl-button btn btn-sm'
+                = f.submit _('Enable'), class: 'gl-button btn btn-sm', data: { confirm: (s_('Runners|You are about to change this instance runner to a project runner. This operation is not reversible. Are you sure you want to continue?') if @runner.instance_type?) }
     = paginate_without_count @projects
 
   .col-md-6

locale/gitlab.pot

@@ -28970,6 +28970,9 @@ msgstr ""
 msgid "Runners|Windows 2019 Shell with manual scaling and optional scheduling. Non-spot. Default choice for Windows Shell executor."
 msgstr ""
 
+msgid "Runners|You are about to change this instance runner to a project runner. This operation is not reversible. Are you sure you want to continue?"
+msgstr ""
+
 msgid "Runners|You can set up a specific runner to be used by multiple projects but you cannot make this a shared runner."
 msgstr ""