[WEBSITE] Update the list of active releases to fetch and display

Summary: The 0.23 is no longer an active release branch, and 0.25 will be soon, so update accordingly.

Test Plan:

bundle exec jekyll serve

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11058