Trunck-6236: Change datatype of onset_date from datetime to date

[njiddasalifu]

Description of what I changed

Change datatype of onset_date from datetime to date

Issue I worked on

https://openmrs.atlassian.net/browse/TRUNK-6236

Checklist: I completed these to help reviewers :)

• [ ] My IDE is configured to follow the code style of this project.

  No? Unsure? -> configure your IDE, format the code and add the changes with git add . && git commit --amend

• [ ] I have added tests to cover my changes. (If you refactored existing code that was well tested you do not have to add tests)

  No? -> write tests and add them to this commit git add . && git commit --amend

• [ ] I ran mvn clean package right before creating this pull request and added all formatting changes to my commit.

  No? -> execute above command

• [ ] All new and existing tests passed.

  No? -> figure out why and add the fix to your commit. It is your responsibility to make sure your code works.

• [ ] My pull request is based on the latest changes of the master branch.

  No? Unsure? -> execute command git pull --rebase upstream master