Specify in \crossref{blockchain} that NU5 is the most recent settled network upgrade.
Specify in \crossref{blockchain} that NU5 is the most recent settled
network upgrade.

Signed-off-by: Daira Hopwood <cd7d43d5556a38ce626d53826fddc46adc597539@jacaranda.org>

zcash/zips,zcash/zips,zcash/zips,zcash/zips