Draft: Add troubleshooting step to update db_key_size

What does this MR do?

Adds a troubleshooting documentation about a problem that can result from !43950 (merged) (discussion) combined with a too short db_key_base.

This aggregates the support team's snippets "Bad Secrets check 2" & "Bad Secret Cleaner".

Related issues

• ZD 148744 (internal) (long-term solution & snippets)
• ZD 183620 (internal)

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