[website] Update donation address to IFP address

Summary: One less address to manage, plus IFP address is publicly verifiable.

Test Plan: Ran it locally, checked links to IFP address and GNC

Reviewers: #bitcoin_abc, PiRK, Fabien

Reviewed By: #bitcoin_abc, PiRK, Fabien

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12600