[Abhinandan Sharma (abhinandan-codes)]

Lower the set_default_build_options() default from C++23 to C++20 and drop the MSVC C++23 switch override. A newer caller-selected C++ standard is still kept.

Why: #471's /std:c++23preview breaks Visual Studio generator builds on CMake < 4.0. CMake can't map it to the C++-only LanguageStandard property, so it lands in AdditionalOptions and reaches C files too. Mixed C/C++ targets (c-util c_util, c_util_reals) then fail with D8016: '/std:c17' and '/std:c++23preview' command-line options are incompatible (ElasticLog RustLib build: CMake 3.31.0, MSVC 19.44). No CMake version maps /std:c++23 (MSVC 19.52+) either. CMake maps C++20 to /std:c++20 (stdcpp20) in every version, so no override is needed.

Local check (MSVC 19.44, mixed C/C++ target + pure C target): CMake 3.31.0 and 4.4.4, Visual Studio and Ninja generators all build. C files get /std:c17 and C++ files /std:c++20. A caller-selected C++23 still builds (stdcpplatest).

Downstream validation: all 27 gates passed in 16 draft PRs on branch abhinandan/validate-cpp20-cbt481, each based on its repo's master. Every c-build-tools pin at every submodule depth is 804727c (for example ElasticLog deps/block-store > MessagingStore validation commit > deps/c-build-tools), every in-graph submodule points to its own validation commit, and pipeline c_build_tools refs match. This PR's own gate also passed.

PR	master base	Result
Azure/macro-utils-c#425	811dd76	Passed
Azure/c-logging#403	580a346	Passed (log_sink_etw_int flaked in 1 of 8 flavors, passed on retry)
Azure/ctest#386	e51c30a	Passed
Azure/c-testrunnerswitcher#387	c3aebb1	Passed
Azure/umock-c#540	d0e9b82	Passed
Azure/c-pal#664	ba3f4b8	Passed
Azure/c-util#697	f4d73e5	Passed
Azure/clds#493	dd4df84	Passed
zrpc !17710036	724a348	Passed
Azure/com-wrapper#321	827164d	Passed
Azure/sf-c-util#405	1e5022f	Passed
Azure-Messaging-Metrics !17710037	e29629c	Passed
Azure-MessagingStore-AITools !17710038	0f4edcd	Passed
Azure-MessagingStore !17710039	44a2f12	Passed: 5/5 (ARM64 LTCG C4702 in C files and a JEMALLOC perf test flaked, both passed on retry)
Azure-Messaging-ElasticLog !17710040	a559371	Passed: 6/6, including Build Rust Library on CMake 3.31.0 + MSVC 19.44
Azure-Messaging-GeoReplication !17710041	244e72c	Passed: 2/2

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[Abhinandan Sharma (abhinandan-codes)]

/azp run

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[Abhinandan Sharma (abhinandan-codes)]

/azp run Azure-C-Build-Tools-Gate

[azure-pipelines]

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