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/controllers/super_admin/app_configs_controller.rb

@@ -39,6 +39,8 @@ class SuperAdmin::AppConfigsController < SuperAdmin::ApplicationController
                          %w[AZURE_APP_ID AZURE_APP_SECRET]
                        when 'email'
                          ['MAILER_INBOUND_EMAIL_DOMAIN']
+                       when 'linear'
+                         %w[LINEAR_CLIENT_ID LINEAR_CLIENT_SECRET]
                        else
                          %w[ENABLE_ACCOUNT_SIGNUP FIREBASE_PROJECT_ID FIREBASE_CREDENTIALS]
                        end