Depreciado: método .json() do Pydantic

[augusto-herrmann]

Ao executar os testes, aparece um warning avisando que o método .json() será depreciado no Pydantic 3.0.

../../usr/local/lib/python3.11/site-packages/pydantic/main.py:1111
../../usr/local/lib/python3.11/site-packages/pydantic/main.py:1111
../../usr/local/lib/python3.11/site-packages/pydantic/main.py:1111
  /usr/local/lib/python3.11/site-packages/pydantic/main.py:1111: PydanticDeprecatedSince20: The `json` method is deprecated; use `model_dump_json` instead. Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.8/migration/
    warnings.warn(

participantes_test.py: 1 warning
plano_entregas/permissions_test.py: 2 warnings
plano_trabalho/permissions_test.py: 1 warning
user_test.py: 8 warnings
  /usr/local/lib/python3.11/site-packages/pydantic/main.py:1087: PydanticDeprecatedSince20: The `dict` method is deprecated; use `model_dump` instead. Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.8/migration/
    warnings.warn('The `dict` method is deprecated; use `model_dump` instead.', category=PydanticDeprecatedSince20)

Alterar a forma de serialização, mudando de .json() para .model_dump_json(), para que no futuro a aplicação possa ser atualizada para usar a versão 3.0 e posteriores do Pydantic.