Establish minimum CMake version 3.15

[mwtoews]

This PR does the following:

• Sets CMake 3.15 as the minimum version
• Cleans up some old logic used to support older version of CMake
• Updates CI testing with a range of versions 3.15 to 3.29

Some rationale for upgrading CMake versions:

• MSVC with Ninja builds repeat "cl : Command line warning D9025 : overriding '/W3' with '/W4'", which makes seeing "real" warnings difficult. This is the same issue and fix described with handle MSVC flag warnings #936. See policy CMP0092, which was attempted with 1df959c, but it seems to only take effect with cmake_minimum_required(VERSION 3.15), thus the message finally goes away!
• CMake 3.15 is over 5 years old, released July 17, 2019. It is also currently the same minimum version used for PROJ and GDAL.
• There are several new features upgrading from 3.13 to 3.15 (e.g. here), but these features are not used in this PR.

Downside:

• This version of CMake is not available as a system package for Linux distros older than Ubuntu 18.04 or Debian 10 (Buster). But this is not a show-stopper on older system, since modern cmake versions can be installed via pip install cmake (using PyPI).

If a compelling case should be made to keep CMake at 3.13, then alternatively:

cmake_minimum_required(VERSION 3.13..3.15)

(docs) can be used to adjust policy settings for 3.15, where feasible.

---

I think this PR is fit to be merged for GEOS 3.13. CMake 3.13 has been the minimum for several releases (since 3.10). However, if deemed too late in the development cycle (or someone really wants version 3.13 for both CMake and GEOS) then hold this merge until after the 3.13 branch is made. I won't mind.

[jmckenna]

thanks @mwtoews ! Indeed I've been applying my PR#996 for a year, so I'm very happy that other packagers are hitting those warnings now, and my voice is heard (through yours ha!). Thanks again.