CMake overwrites Conan VS runtime when CMP0091 is NEW

[czoido]

Changelog: Bugfix: Check the CMP0091 policy and set CMAKE_MSVC_RUNTIME_LIBRARY accordingly to CONAN_LINK_RUNTIME if it's set to NEW.
Docs: omit

Fix #6624

Starting in CMake 3.15, unless you specify cmake_policy(SET CMP0091 OLD) the values that Conan is setting are overwritten.
Now the MSVC_RUNTIME_LIBRARY has to be set, instead of setting the MSVC runtime in CMAKE_<LANG>_FLAGS_<CONFIG>.

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