Observe FLoC changes: "interest-cohort" to "browsing-topics" header

Follow-up to Opt out of FLoC cohort calculation (#327904 - closed) (cc @max-wittig).

A change of the FloC opt-out header is being proposed/discussed in PATCG-individual-drafts/topics. With backwards compatibility for interest-cohort, for now.

If that changes, would we need to change these code locations (naive draft)?

Observing that PATCG draft is probably a good idea.