SCons: Bump required C++ standard to C++17

As per #36436, we now need C++17's guaranteed copy elision feature to
solve ambiguities in Variant.

Core developers discussed the idea to move from C++14 to C++17 as our
minimum required C++ standard, and all agreed. Note that this doesn't
mean that Godot is going to be written in "modern C++", but we'll use
modern features where they make sense to simplify our "C with classes"
codebase. Apart from new code written recently, most of the codebase
still has to be ported to use newer features where relevant.

Proper support for C++17 means that we need recent compiler versions:

 - GCC 7+
 - Clang 6+
 - VS 2017 15.7+

Additionally, C++17's `std::shared_mutex` (conditionally used by
`vk_mem_alloc.h` when C++17 support is enabled) is only available in
macOS 10.12+, so we increase our minimum supported version.

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