Add suggestion about logging

What does this MR do and why?

Adds a documentation about using structured logging for migrations which delete records (and which have potential of data-loss).

• retrospectively this is something what would be useful when restoring data in the recent incident
• also it provides better visibility to what records are deleted (or would be if dry run is used)

Related to https://gitlab.com/gitlab-org/plan/-/issues/492

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