fix(server): stop sending client timestamps

[feugy]

🖖 What's in there?

Part 4 of EXP-3628: Web Analytics events get their timestamp at ingestion time, not from the client.

The server-side track(), trackExposure(), identify() and group() functions stop sending ts. Vercel's ingestion service sets the timestamp when it receives the event.

🤺 How to test?

• pnpm -F @vercel/analytics test: the request body checks in src/server/*.test.ts no longer expect ts.

🔬 Notes to reviewers

• ⚠️ Merge after vercel/api#98450 is deployed. The ingestion service requires ts on the current production version, so it would reject these requests with 400.
• Older versions of the package still send ts. The ingestion service accepts it and ignores it.

🔗 Related PRs

All PRs are independent (not stacked). Deploy them in this order:

1. vercel/api#98449: the subscriber reads the ingestedAt SNS attribute.
2. vercel/api#98450: ingestion sends ingestedAt and ignores client ts.
3. vercel/front#90985: the injected script stops sending ts.
4. This PR: server-side functions stop sending ts.

[vercel]

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