Adds Service Desk custom email

What does this MR do and why?

This MR sets the feature flag default to on for service_desk_custom_email and updates the documentation accordingly.

Contributes to [Feature flag] Rollout of `service_desk_custom_... (#387003 - closed)

Contributes to Configurable e-mail address for service desk (#329990 - closed)

Screenshots or screen recordings

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How to set up and validate locally

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MR acceptance checklist

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