Eventual CMake compatibility break

[ncorgan]

From a recent Windows CI build (CMake 3.23.2):

-- #############################################
-- ## Begin configuration for Python support...
-- #############################################
-- Enabling optional Python bindings if possible...
CMake Deprecation Warning at swig/python/CMakeLists.txt:4 (cmake_minimum_required):
  Compatibility with CMake < 2.8.12 will be removed from a future version of
  CMake.
  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

This line declares CMake 2.8 as the minimum, which I assume is from SoapySDR's beginnings. 2.8.0 is from 2009, and the last tag (2.8.12.2) is from 2014, so even though we list C++11 as the default, we're approaching a possible breaking point.

[zuckschwerdt]

There is a short compatibility overview on https://gitlab.kitware.com/cmake/community/-/wikis/CMake-Versions-on-Linux-Distros
Requiring CMake 3 will bump the OS minimum to e.g. Debian 8 Jessie and Ubuntu 16.04 Xenial.
(I'd say it's reasonable to drop support for stock Wheezy / Trusty.)