Get Envoy Mobile ready for C++20

Unfortunately, it's blocked by the fact that the Android NDK and XCode clang compilers are not C++20 ready. There are some severe enough issues in compilation of C++20 features using Clang 16, that the minimum bar set for upgrade to C++20 for the NDK team (and Google's adoption of XCode for C++20) is Clang 17.

Once we're on an NDK and XCode version that supports Clang 17, we can switch Envoy Mobile to building with C++20.