CI: Fix failures with Ruby 3.5-dev

[Watson1978]

Recently Ruby HEAD has some changes in JSON and it causes failures in tests.
Ref. https://github.com/ohler55/oj/actions/runs/13188779428

1. json_ext_parser_test.rb

Since ruby/ruby@599fbeaffa, it causes failure in test_allocate

=========================================================================================================================================================================================================================================
Failure: test_allocate(JSONExtParserTest):
  [ruby-core:35079].
  <TypeError> exception was expected but none was thrown.
test/json_gem/json_ext_parser_test.rb:14:in 'JSONExtParserTest#test_allocate'
     11:   if defined?(JSON::Ext::Parser)
     12:     def test_allocate
     13:       parser = JSON::Ext::Parser.new("{}")
  => 14:       assert_raise(TypeError, '[ruby-core:35079]') do
     15:         parser.__send__(:initialize, "{}")
     16:       end
     17:       parser = JSON::Ext::Parser.allocate

2. json_generator_test.rb

Since ruby/ruby@89e316ad06, it causes failure in test_fast_state/test_pretty_state/test_safe_state

=========================================================================================================================================================================================================================================
Failure: test_fast_state(JSONGeneratorTest)
test/json_gem/json_generator_test.rb:191:in 'JSONGeneratorTest#test_fast_state'
     188:     actual.delete(:escape_slash)
     189:     actual.delete(:strict)
     190:     actual.delete(:script_safe)
  => 191:     assert_equal({
     192:       :allow_nan             => false,
     193:       :array_nl              => "",
     194:       :ascii_only            => false,
<[[:allow_nan, false],
 [:array_nl, ""],
 [:ascii_only, false],
 [:buffer_initial_length, 1024],
 [:depth, 0],
 [:indent, ""],
 [:max_nesting, 0],
 [:object_nl, ""],
 [:space, ""],
 [:space_before, ""]]> expected but was
<[[:allow_nan, false],
 [:array_nl, ""],
 [:as_json, false],
 [:ascii_only, false],
 [:buffer_initial_length, 1024],
 [:depth, 0],
 [:indent, ""],
 [:max_nesting, 0],
 [:object_nl, ""],
 [:space, ""],
 [:space_before, ""]]>

diff:
  [[:allow_nan, false],
   [:array_nl, ""],
+  [:as_json, false],
   [:ascii_only, false],
   [:buffer_initial_length, 1024],
   [:depth, 0],
   [:indent, ""],
   [:max_nesting, 0],
   [:object_nl, ""],
   [:space, ""],
   [:space_before, ""]]
=========================================================================================================================================================================================================================================

[ohler55]

You are awesome. Thank you.