Docs: update GA4 destination README for dual consent requirement

[tyssejc]

Problem

The GA4 destination code was updated to consent: ["analytics", "marketing"] (AND logic — both required), but the README still shows the old single-category example.

Files to update

• packages/destination-ga4/README.md — update consent example from ["analytics"] to ["analytics", "marketing"]
• Consider adding a note explaining that GA4 requires both analytics_storage and ad_storage/ad_user_data consent signals per Google's consent mode v2 requirements

References

• GA4 Consent Settings
• Google Consent Mode v2