[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