[website] Add note about "ecash=1" default to upgrade notices.

Summary:
It's important for node operators to be aware of this in case they aren't
already setting ecash=1 in the node configuration.

Test Plan: Ran it locally and had a look at the header notice and /upgrade page.

Reviewers: Fabien, bytesofman, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10367