[Feature flag] Rollout of `atlassian_new_app_based_auth_model`

What does this MR do and why?

We're removing the atlassian_new_app_based_auth_model feature flag.

• FF has been released on production
• Testing was successful 🎉

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Checkout the branch
2. Visit the http://gdk.test:3000/-/jira_connect/app_descriptor
3. Verify that new actions are included in the response 🎉

Related to #442334 (closed)