Update univalue subtree

Summary:
This removes the deprecated std::pair wrappers from univalue, so that they are not accidentally re-introduced in our code base.

Backport of Bitcoin Core PR14164
https://github.com/bitcoin/bitcoin/pull/14164

Test Plan:

make check

Reviewers: Fabien, #bitcoin_abc, deadalnix

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4007