Update to legal guidance on use of open source libraries

Why is this change being made?

This MR is being submitted to direct team members to the latest guidance on using open source libraries - this updated guidance will make it easier for team members to determine when legal input is required. Further, this guidance is being moved to live in the legal product handbook, for efficient updating by legal product team members. See related MR

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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