Update deprecated Pydantic v2 methods

[cruisen]

The package currently uses several deprecated Pydantic v2 methods that should be updated to their new equivalents:

1. parse_obj -> model_validate
2. dict -> model_dump
3. __fields_set__ -> model_fields_set

These deprecation warnings appear when using the package with Pydantic v2:

• PydanticDeprecatedSince20: The parse_obj method is deprecated; use model_validate instead.
• PydanticDeprecatedSince20: The dict method is deprecated; use model_dump instead.
• PydanticDeprecatedSince20: The fields_set attribute is deprecated, use model_fields_set instead

These methods will be removed in Pydantic v3.0, so they should be updated to ensure future compatibility.

Relevant files:

• datamodel_code_generator/parser/jsonschema.py

See the Pydantic V2 Migration Guide for more details on the changes.

[riferg206]

@koxudaxi The unit tests for datamodel_code_generator/parser/jsonschema.py also throw deprecation warnings for the same reasons as above. Should these be updated as well or do they need to stay for backwards compatibility with Pydantic V1?