[website] Update the min version to 0.27, display 0.28.x

Summary: This bumps the numbers as 0.26.x is obsolete and 0.28.x coming soon.

Test Plan:

./scripts/fetch_documentation.sh
bundle exec jekyll serve

Check the releases are displayed properly.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14608