[WEBSITE] Fix the download links for the past releases

Summary: The link points to latest, which is invalid for the past releases.

Test Plan:

make doc
bundle exec jekyll serve

Check the download links for the past releases are working.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7779