Reset server projects back to C++20 (#757)

Reset server projects back to C++20 (#757)

* Reset server projects back to C++20

The server projects were on C++23 because we needed C++23's extended modules support (the BMI paths).
We don't use modules anymore for greater compatibility, so we don't need to be on C++23.

Additionally, clang-tidy does not like C++23 (at least, not via stdcpp23 -- CMake uses stdcpplatest which is vaguer but it may pass that through),
so we'll just consistently drop everything back to C++20.

* Include the dependency projects as well

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