Update guidance on using open source software

Why is this change being made?

This change is being made to clarify that the nature of the guidance on using open source software has recently changed, in light on some evident confusion amongst team members.

This MR clarifies that the guidance has shifted to leverage a more comprehensive external resource (license type list and classification system), in contrast to the in-handbook list that was previously published. The use of the more comprehensive resource empowers team members to more efficiently identify what OS licenses are pre-approved by legal for use at GitLab, and which require legal review before use.

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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  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • 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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