Bump OSS setup.py cxx_std to 20 to match fbcode default

Summary:
The OSS `pip install .` build (driven by `setup.py`) pinned `cxx_std=17`,
while the internal Buck2 build uses fbcode's default `-std=c++20`. That
mismatch let C++20 features land in `snapshot_lib/*.cpp` while still
compiling green internally, and only broke later on the public GitHub
Actions runner.

Most recently:

  snapshot_lib/SnapshotReader.cpp:548:
    error: ... has no member named 'starts_with'

`std::string::starts_with` is C++20. The call was introduced by
D103561274 ("Fix SnapshotReader synthetic-filename + relative-path
staging"), passed internal CI on `-std=c++20`, then broke the next OSS
`pip install -v .` because `setup.py` was still pinning `cxx_std=17`.

Bumping the OSS knob to C++20 lines the two builds up so we don't
have to chase a workaround at every new C++20 call site.

pybind11's `cxx_std=20` requires GCC >= 10, Clang >= 10, or MSVC>= 19.20, which the GitHub Actions `ubuntu-latest` image and the
runners we publish all already meet.

Reviewed By: pdepetro

Differential Revision: D104728867

fbshipit-source-id: 8db44407ccb0cb6ed7486012cf276148b85b646e

Author: bvu405

setup.py

@@ -67,7 +67,7 @@ def find_zstd():
         include_dirs=["."] + zstd_include_dirs,
         library_dirs=zstd_library_dirs,
         libraries=["zstd"],
-        cxx_std=17,
+        cxx_std=20,
         define_macros=[("PYBIND11_DETAILED_ERROR_MESSAGES", None)],
     ),
 ]