[website] Show an "Avalanche-enabled" banner in the release page

Summary:
Make it clear that 0.26.x release are suitable for avalanche.

Test Plan:

bundle exec jekyll serve

Massage your cache dir and release json, then check the banner is displayed with a 0.26.0 release. Removing the 0.26.x releases restore the previous "eCash compatible" banner.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11938