[smart_holder] Remove obsolete `detail::type_info::default_holder` member. (#5541)
* git merge --squash purge_internals_versions_4_5
* Remove PYBIND11_HAS_INTERNALS_WITH_SMART_HOLDER_SUPPORT, set PYBIND11_INTERNALS_VERSION 7
* Remove all uses of PYBIND11_SMART_HOLDER_ENABLED under include/pybind11
* Remove obsolete PYBIND11_ACTUALLY_USING_SMART_HOLDER_AS_DEFAULT macro.
* Remove PYBIND11_SMART_HOLDER_ENABLED in ubench/holder_comparison.cpp
* Remove all uses of PYBIND11_SMART_HOLDER_ENABLED under tests/
* Remove `#define PYBIND11_SMART_HOLDER_ENABLED`
* Remove all uses of PYBIND11_SMART_HOLDER_TYPE_CASTERS under tests/
* Remove all uses of PYBIND11_TYPE_CASTER_BASE_HOLDER under tests/
* Add missing `#include <cstdint>`
Example error message (🐍 3.11 • ubuntu-latest • x64, GNU 13.3.0):
```
include/pybind11/detail/value_and_holder.h:56:52: error: ‘uint8_t’ is not a member of ‘std’; did you mean ‘wint_t’?
56 | inst->nonsimple.status[index] &= (std::uint8_t) ~instance::status_holder_constructed;
| ^~~~~~~
```
* Remove type_info::default_holder member. DOES NOT BUILD
* Remove some obsolete default_holder code and #ifdef out uses of typeinfo->default_holder. BUILDS BUT 2 TESTS ARE FAILING.
* Replace `default_holder` with `holder_enum_v == holder_enum_t::std_unique_ptr`
Intentionally not changing error messages, because this would result in a significantly bigger change.
* Change PYBIND11_INTERNALS_VERSION to 106: It will be changed to 7 in a follow-on PR that actually changes the internals.
* Change PYBIND11_INTERNALS_VERSION to 7 (because this PR actually changes the internals).4 files changed
