Accept NEW CMake policies up to CMake 3.14

[craigscott-crascit]

Starting with CMake 3.27, deprecation warnings are issued
when asking for policy settings for CMake 3.4 or earlier.
The cmake_minimum_required() command accepts a version
range, which allows NEW policy settings up to the upper end
of that range to be used, but without raising the minimum
CMake version above the bottom of that range. This means
NEW policy settings will be used where available, without
requiring them. This change updates the project's
cmake_minimum_required() calls to use a version range to
extend the upper policy version to 3.14 where it wasn't already
at that version or higher. This prevents the deprecation warning
from CMake 3.27, and gives breathing space before a future
CMake release will start issuing similar deprecation warnings
again.

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for this kind of bug). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[pboettch]

Related: #4076

[nlohmann]

Please update from the develop branch which now has a working CI (again).

[craigscott-crascit]

Rebased as requested.

[coveralls]

coverage: 100.0%. remained the same when pulling 4b462d0 on craigscott-crascit:cmake-policies-update into 6d4b72d on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks for the patience!