CMake version 3.6 is getting outdated.

[christianbrugger]

When configuring range-v3 with the new cmake 3.31 I get the following warning:

CMake Deprecation Warning at external/range-v3/CMakeLists.txt:8 (cmake_minimum_required):
  Compatibility with CMake < 3.10 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.


-- [range-v3]: C++ std=latest

As stated in #1724 the actual needed feature set of the CMakeLists.txt seems to be at least 3.15. In this case I suggest to change this line

cmake_minimum_required(VERSION 3.6)

simply to

cmake_minimum_required(VERSION 3.15)

This would also close #1724