Docs: Describe 17.3.1 one-time SAST cleanup migration

What does this MR do and why?

These details were originally added in Docs: Describe 17.3 one-time SAST cleanup migra... (!158872 - merged) • Adam Cohen, Connor Gilbert+ • 17.3, then removed in Remove docs update for SAST cleanup migration (!162693 - merged) • Adam Cohen • 17.4 because of a last-minute bug that prevented the migration from being released in %17.3. Now that the bug has been fixed and backported to 17.3.1, we can update the docs to describe this migration.

Related issues

• Create data migration to automatically resolve ... (#444926 - closed) • Adam Cohen • 17.4 • Needs attention
  • MR: Add migration to resolve vulnerabilities from r... (!157776 - merged) • Adam Cohen • 17.3

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