Refactors managing managers requirement to a required capability

Why is this change being made?

Providing career progression for our team and aligning more closely to requirements for this level across GitLab and the industry, we're refactoring the requirement for Senior Engineering Manager to manage managers into a required capability. While a Senior Engineering Manager may not be managing managers at all times, they should have the capability and willingness to manage managers as is required by the organization. We have also added a few additional criteria to ensure the bar for this level is maintained high.

Author Checklist

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