Property value "average" should work for person properties or disallow them

Bug Description

Bug description

The property average math for a trends insight with a person property looks at event properties instead of person properties.

How to reproduce

1. Create a new trends insight
2. Select "Property value [average]" as math
3. Select a person property
4. Check the SQL in the debug tab and see that we're using an event property

Additional context

https://posthoghelp.zendesk.com/agent/tickets/24638 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/28853)

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables or https://eu.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

Reproduce the trends insight described in the issue, selecting “Property value [average]” with a person property. Inspect the SQL in the debug tab and trace where the property source is selected; done means the average uses the person property or the UI disallows this combination.