Change C++ standard to C++17.

[hysw]

C++20 is not well support with all our compilation environments.

[hysw]

So some additional context on the C++ standard version:

• Our presubmit runs on clang 10 (Linux) and 12 (Android NDK23), so C++20 is not particularly well supported. (e.g. NDK issue 1886 )
• We got very little benefit from C++20, there's around 30 lines of code uses C++20 features, and non of their functionality actually requires C++20 features.
• We've already spend hours dealing with C++20 related issues ( Update googletest submodule to latest #210 ; Move C++20 feature out of the header file. #262 ; Add --config-json-path to accept flags specified in a JSON file. #265 ; merging patch from internal upstream. #266 )
• In addition, other Google open source projects uses up to C++17.

[Keenuts]

Fine with me, since c++20 is causing more issues than it brings benefits for now.
(I largely prefer this to local patches here and there to be c++17 compliant).

[chaoticbob]

Can you add a line to main README.md that says something along the lines of C++17 compliant compiler? Maybe a bullet point under the Requirements?

[hysw]

Can you add a line to main README.md that says something along the lines of C++17 compliant compiler? Maybe a bullet point under the Requirements?

Added to both README.md and CONTRIBUTING.md

• README.md Linux requirement: clang 10 is our presubmit, and gcc 9 is fully C++17 compliant
• README.md MSVC 2019 can compile up to C++20, so it's a higher requirement than a C++17 compliant compiler
• CONTRIBUTING.md since that's where submission requirement is listed

[chaoticbob]

LGTM