[CMAKE] Bump minimum cmake to 3.16
This is the version from Debian Buster backports. It fixes a couple
issues, and it required for some more coming features (e.g. D6751).
Depends on D7072 to prevent to Gitian builds from failing.
Read the docs and try the instructions.
cmake -GNinja .. ninja all check-all
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D7073