Fix deprecation warnings for json legacy reader

[davidwendt]

Description

Fixes deprecation warnings caused by changes in #15558
Most are in the json_test.cpp and appear like this

[150+7+50=206] Building CXX object tests/CMakeFiles/JSON_TEST.dir/io/json_test.cpp.o
/cudf/cpp/tests/io/json_test.cpp: In member function 'virtual void JsonReaderParamTest_BasicJsonLines_Test::TestBody()':
/cudf/cpp/tests/io/json_test.cpp:320:14: warning: 'cudf::io::json_reader_options_builder& cudf::io::json_reader_options_builder::legacy(bool)' is deprecated [-Wdeprecated-declarations]
  317 |     cudf::io::json_reader_options::builder(cudf::io::source_info{data.data(), data.size()})
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  318 |       .dtypes(std::vector<data_type>{dtype<int32_t>(), dtype<double>()})
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  319 |       .lines(true)
      |       ~~~~~~~~~~~~
  320 |       .legacy(is_legacy_test(test_opt));
      |       ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /cudf/cpp/tests/io/json_test.cpp:30:

Compiler warnings usually result in errors when building libcudf.

This PR removes calls and references to legacy JSON reader features where possible.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

Thank you @davidwendt!

[bdice]

I'll add a follow-up item to the tracking issue to remove the word "experimental." I'll tackle that after this merges. #15537

[bdice]

/merge