AVRO-4271: [ruby] Use json instead of multi_json

[ericproulx]

What is the purpose of the change

This resolves AVRO-4271 by switching the Ruby runtime dependency from
multi_json to the json gem and updating Ruby JSON parsing/generation calls
to use JSON directly. It also removes the stale Yajl.dump call in protocol
message serialization.

The parsing calls do not pass the obsolete quirks_mode option or an explicit
create_additions: false, because quirks_mode has not been needed since
json 2.0 and JSON.parse disables additions by default.

This also adds bigdecimal as an explicit dependency because it is no longer a
default gem as of Ruby 3.4.

Verifying this change

This change is already covered by existing tests. The following targeted tests
were run locally:

• ruby -Ilib:test test/test_schema.rb
• ruby -Ilib:test test/test_schema_normalization.rb
• ruby -Ilib:test test/test_protocol.rb
• ruby -Ilib:test test/test_datafile.rb --name test_differing_schemas_with_complex_objects
• ruby -Ilib:test test/test_io.rb --name '/test_(null|boolean|string|bytes|int|long|float|double)/'

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable