[e.cash] Use constant for latest version on upgrade page

Summary: So we only have to change the version in one place next time.

Test Plan: Ran it locally, looks fine

Reviewers: johnkuney, #bitcoin_abc, bytesofman

Reviewed By: johnkuney, #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14721