Make Posthog API Host configurable in flowforge.yml (backport #1649)

[github-actions]

Backport of #1649

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Description

We had hard-coded the PostHog API host to be https://app.posthog.com. We are migrating our data to a different host, and so this needs to be configurable through the flowforge.yml.

Related Issue(s)

• Helm Update Issue: ConfigMap Update: PostHog API Host helm#106
• CloudProject Issue: https://github.com/flowforge/CloudProject/issues/133

Checklist

• I have read the contribution guidelines
• [-] Suitable unit/system level tests have been added and they pass
• Documentation has been updated
• [-] Upgrade instructions
• Configuration details
• [-] Concepts
• Changes flowforge.yml?
• Issue/PR raised on flowforge/helm to update ConfigMap Template
• Issue/PR raised on flowforge/CloudProject to update values for Staging/Production

Labels

• Backport needed? -> add the backport label
• [-] Includes a DB migration? -> add the area:migration label