Tweak security operating principles to refer to dogfooding

Why is this change being made?

In !127639 (merged) we updated our Security Mission, Vision and Operating Principles after the security leadership offsite. One point I'd like to discuss:

• Evangelize creative GitLab solutions publicly

This reads a bit to me as it's meaning to follow dogfooding. But reading creative here suggests to me any use of GitLab for the sake of using GitLab is good . This however is not the case as clarified recently in gitlab-com/content-sites/handbook!155 (merged) so we might want to change the wording here slightly to conform with our idea of dogfooding.

Author Checklist

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• 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
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