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.

.env.example

@@ -155,10 +155,6 @@ TWITTER_ENVIRONMENT=
 SLACK_CLIENT_ID=
 SLACK_CLIENT_SECRET=
 
-#Linear Integration
-LINEAR_CLIENT_ID=
-LINEAR_CLIENT_SECRET=
-
 # Google OAuth
 GOOGLE_OAUTH_CLIENT_ID=
 GOOGLE_OAUTH_CLIENT_SECRET=