docs(sm) - update secrets extraction for version 8.5

[upgradingdave]

Description

There is no longer a camunda-postgresql secret as of 8.5. Here's a suggestion to improve this page:

https://docs.camunda.io/docs/self-managed/setup/upgrade/#secrets-extraction

I also updated the Note. Many of the components are customizable, and so this secrets extraction script will be different for each environment. So, rather than adding comments such as ("uncomment if using console"), I suggest we give a script including everything with a Note to remind folks to customize it as needed.

When should this change go live?

• [ ] This is a bug fix, security concern, or something that needs urgent release support.
• [ ] This is already available but undocumented and should be released within a week.
• [ ] This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• [ ] This is part of a scheduled alpha or minor. (apply alpha or minor label)
• [x] There is no urgency with this change and can be released at any time.

PR Checklist

• [x] My changes are for an already released minor and are in /versioned_docs directory.
• [x] My changes are for the next minor and are in /docs directory (aka /next/).

• [x] My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• [ ] My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.