[website] Update the link in the release page to the avalanche blog post

Summary: Similar to what is done on D12127 and D12128, update the link in the release banner. Also cleanup the no longer needed activation mechanism.

Test Plan:

bundle exec jekyll serve

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12132