[website] Add info for people who forgot to upgrade their node

Summary:
since 0.23.4 has a post-upgrade checkpont, it will automatically reconsider that block upon startup.
This fixes invalid-marked block for people who left an old node running though the upgrade.

Test Plan: Built the site and looked at it. Checked the link.

Reviewers: #bitcoin_abc, josephroyking

Reviewed By: #bitcoin_abc, josephroyking

Differential Revision: https://reviews.bitcoinabc.org/D9535