[website] Update upgrade page for completed upgrade

Summary: Similar to D12498

Test Plan:
Read it through to check info is correct.
Verify that previous block (792116) is above upgrade MTP, and the one before that (792115) is below.
Ran the site locally and checked links

Reviewers: PiRK, #bitcoin_abc

Reviewed By: PiRK, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13899