Remove start_date from team member file instructions and 3 files

Why is this change being made?

Starting around 2021-06, we stopped treating "start date" as public information. As a result, the start_date line was removed from all team member data files. However, we forgot to remove it from the instructions, where it was still listed as a required line. Some recently-created files included this line.

This removes the start_date line from all files that have it (3) and removes the start_date line from the instructions.

Note: This should solve the problem, but if we continue to see start_date appear in these files for some reason we could add a validation check. See Commit 74f838b8 for inspiration.

Author Checklist

• Provided a concise title for the 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 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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