Move pipeline vulnerability report store to the entry point

What does this MR do and why?

This moves the pipeline vulnerability store's initialization to the app's entry point. The store is then set up in the app's root component. This ensures that the app can seamlessly register its store modules and plugins no matter where/how the store instance has been created. This makes it easier for us to migrate this Vue app to the unified pipeline tabs app that will register a single store to which each tab app should attach its own modules.

Screenshots or screen recordings

No visual changes as this is only backstage.

How to set up and validate locally

1. Run a pipeline in a project that has some security scans set up.
2. Navigate to the pipeline's page.
3. Activate the Security tab.
   • The app should work as usual.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #363077 (closed)