Migrated events do not have $groups property set correctly

Bug Description

• User cannot filter dashboard by group. Wants to ideally filter by group name.
• User imported historical events that did not get properly fed into the database.
• When filtering by group name in the dashboard, it only applies to the new events, not the historical ones.

Some of the migrated events did not populate $group_0 field in the events, as they have properties.groups set instead of properties.$groups.

Zendesk tickets:

• https://posthoghelp.zendesk.com/agent/tickets/42355 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/42647)
• https://posthoghelp.zendesk.com/agent/tickets/41715 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/58526)
• https://posthoghelp.zendesk.com/agent/tickets/41842 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/42646)

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]

No file, test, or entry point is named. Start by tracing how migrated events store properties.groups versus properties.$groups, then verify how dashboard group filters read those properties. Done means historical events populate $group_0 consistently and filtering by group name includes both historical and new events.