Add Gitlab Advanced SAST (Experiment Mode) in ULTIMATE_FEATURES

What does this MR do and why?

Add advanced_sast to GITLAB_FEATURES, for enabling the new GitLab Advanced SAST analyzer for Ultimate customers.
It should be available to Ultimate customers only, however we include the CTA within GitLab Core (hence no EE: true)

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

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Before	After

How to set up and validate locally

1. Ensure flightjs/Flight project is seeded locally
2. Visit Security Configuration page to see GitLab Advanced SAST CTA http://gdk.test:3000/flightjs/Flight/-/security/configuration

Related to #465148