[website] Change Avalanche link on upgrade banner

Summary: Direct people to the the Post-Consensus blog post, which has more general information than the Staking tutorial.

Test Plan: Ran it locally, checked the link.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12128