Adds mailroom troubleshooting guide for 16.6.0

What does this MR do and why?

Contributes to #432257 (closed)

Adds mailroom troubleshooting guide for 16.6.0.

Email ingestion doesn't work in 16.6.0 due to Extend encrypted secrets support to Redis (!136002 - merged) and there is a manual patch available. 16.6.1 will fix this issue.

Added the troubleshooting to both incoming email and Service Desk documentation as both features are affected and are used independently by customers.

Screenshots or screen recordings

🚫 documentation only

How to set up and validate locally

🚫 documentation only

MR acceptance checklist

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