Bump CMake minimum version to 3.18

Summary:
This makes it possible to find the Python::Development.Embed target that has been introduced in this version. All distros ship a higher version by default, Debian bullseye being the oldest one with 3.18.

While I'm touching it, I improved the python search as well to only include what is needed, enforce the minimum version, make it mandatory and remove the redundant log.

Test Plan:

cmake -GNinja .. -DBUILD_BITCOIN_CHRONIK=ON -DBUILD_BITCOIN_CHRONIK_PLUGINS=ON
ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16946