grpc: refactor cxxStandard selection

GRPC is less aggressive in overriding `CMAKE_CXX_STANDARD` nowadays.
This allows us to take a less invasive approach to ensure that the
provided abseil package is configured with the same implementation
for the provided c++17 compatibility shims.
Instead of always setting `CMAKE_CXX_STANDARD`, we only do it to
override hard-coded downgrade on darwin. With that, we can use the
default configuration for abseil-cpp, reducing the number of instances
of that library in the build closure to one.

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