[Java] NullableDateMilliVector.getObject() should return a LocalDate, not a LocalDateTime

NullableDateMilliVector.getObject() today returns a LocalDateTime. However, this vector is used to store date information, and thus, getObject() should return a LocalDate.

Please note: there already exists a vector that returns LocalDateTime –
the NullableTimestampMilliVector.

**Reporter**: [Vanco Buca](https://issues.apache.org/jira/browse/ARROW-1984)
#### PRs and other links:
- [GitHub Pull Request apache/arrow#5315](https://github.com/apache/arrow/pull/5315)

_{**Note**: *This issue was originally created as [ARROW-1984](https://issues.apache.org/jira/browse/ARROW-1984). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Comienza en NullableDateMilliVector.getObject() y compara su comportamiento con NullableTimestampMilliVector, que el issue identifica como la implementación de LocalDateTime. Confirma el pull request existente de GitHub y el contexto de migración antes de realizar cambios; el trabajo estará terminado cuando el vector de fechas devuelva LocalDate, mientras que el vector de marcas de tiempo siga siendo distinto.

Escrito por el modelo de indexación a partir del texto del issue.