[website] Update the upgrade page for the next release

Summary: This includes instructions to upgrade the node. Since 0.28.0 is not available yet it required to have a min version filter so it displays properly.

Test Plan:

bundle exec jekyll serve --incremental

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14618