fix(ntp): align cpp17 guard macro

[NewYaroslav]

Motivation

• Ensure the NTP time service uses the public C++17 detection macro so singleton storage selection is consistent across build configurations.
• Replace a private guard that could diverge from the project's public feature macro to avoid mismatched definitions between declaration and definition.

Description

• Replaced occurrences of the private _TIME_SHIELD_CPP17 guard with the public TIME_SHIELD_CPP17 in include/time_shield/ntp_time_service.hpp.
• The change unifies the conditional extern/definition and the inline/static singleton paths to follow the same standard check.

Testing

• Built and ran tests for C++11 with cmake -S . -B build-cpp11 -DCMAKE_CXX_STANDARD=11 -DTIME_SHIELD_CPP_BUILD_TESTS=ON, cmake --build build-cpp11 and ctest --test-dir build-cpp11, and all tests passed.
• Built and ran tests for C++14 with cmake -S . -B build-cpp14 -DCMAKE_CXX_STANDARD=14 -DTIME_SHIELD_CPP_BUILD_TESTS=ON, cmake --build build-cpp14 and ctest --test-dir build-cpp14, and all tests passed.
• Built and ran tests for C++17 with cmake -S . -B build-cpp17 -DCMAKE_CXX_STANDARD=17 -DTIME_SHIELD_CPP_BUILD_TESTS=ON, cmake --build build-cpp17 and ctest --test-dir build-cpp17, and all tests passed (100% of tests).

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Codex Task