feat: implement updatePost endpoint and modify datePosted handling

[mitchell-liatrio]

This pull request introduces several changes to the PostController and related files to add support for updating posts via a new PUT endpoint. Additionally, it includes modifications to the Post model and updates to the test suite to cover the new functionality.

Controller changes:

• Added a new PUT endpoint to update posts in PostController with validation and error handling for date parsing and missing posts.

Model changes:

• Changed the setDatePosted method in the Post model to accept a String instead of a Date to align with the new update functionality.

Test changes:

• Added new test cases in PostControllerTest to cover successful updates, not found errors, and bad request errors for the new PUT endpoint.
• Imported necessary Mockito and Spring test utilities in PostControllerTest to support the new test cases.