[smart_holder] Remove obsolete detail::type_info::default_holder member.

[rwgk]

Description

The core change under this PR is (include/pybind11/detail/internals.h):

@@ -234,6 +234,7 @@ struct type_info {
     buffer_info *(*get_buffer)(PyObject *, void *) = nullptr;
     void *get_buffer_data = nullptr;
     void *(*module_local_load)(PyObject *, const type_info *) = nullptr;
+    holder_enum_t holder_enum_v = holder_enum_t::undefined;
     /* A simple type never occurs as a (direct or indirect) parent
      * of a class that makes use of multiple inheritance.
      * A type can be simple even if it has non-simple ancestors as long as it has no descendants.
@@ -241,13 +242,8 @@ struct type_info {
     bool simple_type : 1;
     /* True if there is no multiple inheritance in this type's inheritance tree */
     bool simple_ancestors : 1;
-    /* for base vs derived holder_type checks */
-    // SMART_HOLDER_BAKEIN_FOLLOW_ON: Remove default_holder member here and
-    // produce better error messages in the places where it is currently used.
-    bool default_holder : 1;
     /* true if this is a type registered with py::module_local */
     bool module_local : 1;
-    holder_enum_t holder_enum_v = holder_enum_t::undefined;
 };

This PR

• only resolves the "Remove default_holder member" part (see above), but
• does not make changes to "produce better error messages".

Rationale:

• Removing the default_holder member requires a PYBIND11_INTERNALS_VERSION bump. To minimize bumps on master, this is done before merging smart_holder into master.
• The changes to produce better error message are assumed to be significantly larger than this conservative PR, and they will change the pybind11 behavior. These changes are best done with reviews on the master branch (before the pybind11 v3.0.0 release).

// SMART_HOLDER_BAKEIN_FOLLOW_ON: Refine holder compatibility checks. comments are added here, so that it will be easy to pin-point where we want to produce better error messages.

Suggested changelog entry:

[rwgk]

@rhaschke for visibility. — I hope this will bring an end to the smart_holder / PYBIND11_INTERNALS_VERSION confusion.