[website] Show 0.29.x versions on the release page

Summary: And stop showing 0.27.x which are deprecated.

Test Plan:

./scripts/fetch_documentation.sh
bundle exec jekyll serve

0.27.x versions are no longer displayed.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15851