[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.*}

从 NullableDateMilliVector.getObject() 入手，并将其行为与 NullableTimestampMilliVector 进行比较，issue 将后者标识为 LocalDateTime 的实现。在进行更改之前，确认现有的 GitHub pull request 和迁移上下文；完成的标准是日期向量返回 LocalDate，而时间戳向量保持独立。

由索引模型根据 Issue 内容生成。