Merge #5741 and make keep the default error_handler of the binary writers

Merges claude/binary-utf8-roundtrip-5651, where the writers now only check
UTF-8 if JSON_STRICT_BINARY_UTF8 is enabled, and applies the same rule to
the error_handler parameter added here: to_cbor(), to_ubjson(),
to_bjdata(), and to_bson() default to error_handler_t::keep, the behavior
of 3.12.0, or to error_handler_t::strict if JSON_STRICT_BINARY_UTF8 is
enabled (detail::binary_writer_default_error_handler()). Passing a handler
explicitly always takes precedence.

- Conflicts in binary_writer.hpp resolved in favor of this branch's
  error_handler, which replaces #5741's check_text_utf8().
- Tests: the "default parameters" section checks that the default equals
  keep; unit-binary_utf8_strict.cpp checks that an explicit handler
  overrides the macro's default.
- Docs: signatures show keep; parameter, exception, and format pages
  describe keep as the default and the macro; the two 3.13.0 version
  history entries of each writer are merged into one.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/output/error_handler.hpp

@@ -34,5 +34,17 @@ enum class error_handler_t
     keep     ///< keep invalid UTF-8 sequences unchanged
 };
 
+/// the default error handler of the CBOR, UBJSON, BJData, and BSON writers:
+/// error_handler_t::strict if JSON_STRICT_BINARY_UTF8 is enabled, otherwise
+/// error_handler_t::keep (the behavior before version 3.13.0)
+constexpr error_handler_t binary_writer_default_error_handler() noexcept
+{
+#if JSON_STRICT_BINARY_UTF8
+    return error_handler_t::strict;
+#else
+    return error_handler_t::keep;
+#endif
+}
+
 }  // namespace detail
 NLOHMANN_JSON_NAMESPACE_END