Add test for libstdc++

[nlohmann]

• add CI job to use libdc++ to detect errors like tests fail to build with clang-19 and libc++ due to unsupported std::char_traits #4490
• remove unsupported char types (see tests fail to build with clang-19 and libc++ due to unsupported std::char_traits #4490)
• update lukka/get-cmake to fix warning
• add python-version for actions/setup-python@v5 to fix warning

[coveralls]

coverage: 100.0%. remained the same
when pulling e9eadbc on libcxx
into 0604140 on develop.

[nlohmann]

Thanks @paparodeo for the analysis and the fix!