[website] Make the 0.28.x releases obsolete

Summary: Stop building the docs for these versions and stop showing them on the release page. Also prepare for the 0.30.x releases to come.

Test Plan:

bundle exec jekyll serve --incremental

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16828