[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.*}

從 NullableDateMilliVector.getObject() 開始，並將其行為與 NullableTimestampMilliVector 進行比較，issue 將後者識別為 LocalDateTime 的實作。在進行變更之前，確認現有的 GitHub pull request 和遷移背景；完成的條件是日期向量回傳 LocalDate，而時間戳記向量維持獨立。

由索引模型根據 Issue 內容生成。