Updated "How to Engage with Product Management" to replace the term "Technical Account Manager" with the now used CSM and link to their responsibilities

Why is this change being made?

GitLab does not employ anymore Technical Account Manager and my new joining colleagues, other CSMs, are confused about the job title TAM being used but not present in the Account Team. This MR replaces the title one to one to the now used Customer Success Manager title.

Author Checklist

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