Add documentation on how to update the upstream scanner
What does this MR do?
Adds documentation on how to update the upstream scanner.
Why are we doing this?
In order to have clear instructions on how to update the upstream scanner
What are the relevant issue numbers?
Composition Analysis - Reaction Rotation for 16.10 (gitlab-org/secure/general#297 - closed)
Also relates to https://gitlab.com/gitlab-org/security-products/release/-/merge_requests/130#note_1783454067