C++ 20: Move vnext to C++ 20 and C++ 20 coroutines

[NickGerleman]

This moves projects in vnext to build with C++ 20 and C++ 20 coroutines. This should not impact the ABI of MSRN DLLs, but may need care for cases of Folly at legacy ABI boundary.

1. Set <CppStandard>stdcpp20</CppStandard> in React.cpp.props
2. Remove explicit usages of /await to opt into coroutines TS
3. Replace std::experimental::coroutine_handle with std::coroutine_handle

This depends on the earlier changes being merged, along with the rest of the std::future usages as a coroutine return type replaced (e.g. with winrt types).

Also needed to bump Folly version due to facebook/folly@d62707b.

Microsoft Reviewers: Open in CodeFlow

[NickGerleman]

@vmoroz after the other PRs are merged, I think the only thing we have left to do before this is to convert a few remaining usages of std::future, not addressed in #12327.

I'm going to leave this for you all to be able to pick up the rest (I do not have a well-used Windows dev machine, which has made this change a bit annoying). I think the bulk of the work should be done though.

[vmoroz]

@vmoroz after the other PRs are merged, I think the only thing we have left to do before this is to convert a few remaining usages of std::future, not addressed in #12327.

I'm going to leave this for you all to be able to pick up the rest (I do not have a well-used Windows dev machine, which has made this change a bit annoying). I think the bulk of the work should be done though.

@NickGerleman , thank you for implementing the change!

[chiaramooney]

/azp run

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