Update terminology to use Incident Manager instead of IMOC

Why is this change being made?

This MR shifts our terminology to use "Incident Manager" or "Incident Manager On Call" instead IMOC.

Incident Manager On Call (IMOC) does not make sense in many contexts as it conflates the role of Incident Manager with on-call responsibilities.

For example, it is common to have actively engaged Incident Managers who are not currently on call.

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