fix(web-modeler): correct environment variables for webapp certificate

[wollefitz]

Description

Corrects the environment variables for configuring a SSL certificate for Web Modeler's webapp component, see Slack discussion.

Relates to camunda/web-modeler#9730

When should this change go live?

• [x] 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)
• [ ] 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.

[akeller]

This is a bug fix, security concern, or something that needs urgent release support.

👀 Ping us next time via #ask-c8-documentation. I think we were all sleeping when this got merged 😆 I'll do a docs release now.