feat: move Linear config to installation_config (#10999)

This PR has the following changes

- Add `LINEAR_CLIENT_ID` and `LINEAR_CLIENT_SECRET` to installation
config
- Add Linear config to super_admin/features.yml
- Replace usage of ENV.fetch with GlobalConfigService.load for fetch
Client Id and Secret.

app/models/integrations/app.rb

@@ -55,9 +55,10 @@ class Integrations::App
   end
 
   def build_linear_action
+    app_id = GlobalConfigService.load('LINEAR_CLIENT_ID', nil)
     [
       "#{params[:action]}?response_type=code",
-      "client_id=#{ENV.fetch('LINEAR_CLIENT_ID', nil)}",
+      "client_id=#{app_id}",
       "redirect_uri=#{self.class.linear_integration_url}",
       "state=#{encode_state}",
       'scope=read,write',