Docs: Update how to preview SAST deprecations

What does this MR do?

This updates the preview steps from !95656 (merged) to:

• simplify the steps
• reflect the specific revision necessary for testing in Self-Managed instances due to the change missing the 15.3 self-managed package by a few hours (gitlab-com/www-gitlab-com!109377 (comment 1070883042))

My thought is that we'd update the RP item to point to this specific section for instructions. It is important to note that the existing RP item content is NOT misleading for GitLab.com users, who can directly import SAST.latest. It is only slightly off for Self-Managed since the template isn't updated in the 15.3 package.

Related issues

#366242 (closed)

gitlab-com/www-gitlab-com!109377 (merged)

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