googletest

cmake: Raise min to 3.6

From the CMake 3.27 release notes:
Compatibility with versions of CMake older than 3.5 is now
deprecated and will be removed from a future version. Calls to
cmake_minimum_required() or cmake_policy() that set the policy
version to an older value now issue a deprecation diagnostic.

This PR also removes manually setting policy CMP0048. This is
redundant since the CMake min is already 3.X