[website] Update the upgrade page and banner

Summary: Update for the may 15, 2024 upgrade.

Test Plan:

bundle exec jekyll serve

Try changing the activation time to see the banner in the release page.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15923