Document publishing EPSS data to new bucket rather than new dir

What does this MR do and why?

Following the discussion in Introduce a sync mechanism for EPSS scores (#467672 - closed) • Yasha Rise • 17.5 • On track and the concerns raised regarding implementation (see #467672 (comment 1983463426)), we decided it's best the MVC proceeds with a new bucket for EPSS rather than using the existing PMDB advisories bucket.

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

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.