[smart_holder] Fix terrible merge accident: `PYBIND11_INTERNALS_SH_DEF` missing in `PYBIND11_INTERNALS_ID` and `PYBIND11_MODULE_LOCAL_ID`

[rwgk]

Description

This PR fixes two oversights:

1. A terrible merge accident introduced with pybind/pybind11#5085: PYBIND11_INTERNALS_SH_DEF was accidentally removed from PYBIND11_INTERNALS_ID and PYBIND11_MODULE_LOCAL_ID. — How did this happen? First PR #5084 on master introduced whitespace changes:

• https://github.com/pybind/pybind11/pull/5084/files#diff-b47e807c57dacc02a52aa80c712d77051f145748c3be9b8c4e90dd1aa27f7e34

Those were incorrectly applied in smart_holder PR #5085:

• https://github.com/pybind/pybind11/pull/5085/files#diff-b47e807c57dacc02a52aa80c712d77051f145748c3be9b8c4e90dd1aa27f7e34

2. Simple change that was overlooked before: missing -DCMAKE_CXX_FLAGS="-DPYBIND11_USE_SMART_HOLDER_AS_DEFAULT" (manylinux)

Validation of both fixes:

ci.yml manylinux:

C++ Info: 13.2.1 20231014 C++17 __pybind11_internals_v5_gcc_libstdcpp_cxxabi1018__ PYBIND11_SIMPLE_GIL_MANAGEMENT=False PYBIND11_NUMPY_1_ONLY=False

ci_sh_def.yml manylinux:

C++ Info: 13.2.1 20231014 C++17 __pybind11_internals_v5_gcc_libstdcpp_cxxabi1018_sh_def__ PYBIND11_SIMPLE_GIL_MANAGEMENT=False PYBIND11_NUMPY_1_ONLY=False

Suggested changelog entry: