[CMAKE] Bump minimum cmake to 3.16

Summary:
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.

Test Plan:
Read the docs and try the instructions.

cmake -GNinja ..
ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7073