[WEBSITE] Add an upgrade notice banner to the release page

Summary: This will display an upgrade notice before and 1 week after the date.

Test Plan:

make serve

Go to the release page and check the result.

Reviewers: #bitcoin_abc, josephroyking, Mengerian

Reviewed By: #bitcoin_abc, josephroyking, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D9336