Fix date conversion on the server-side

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes #16795 (and #16779)

Description

The DateTimeOffset parsing added in #16732 is the root cause of both #16795 and #16779. #16779 was fixed by a client-side change in the 14.1.1 patch, but #16795 still happens in that patch.

By the looks of it, any "date without time" property value saved before 14.1 will not work in 14.1 and 14.1.1 😱

This PR fixes the problem at server level by:

1. Handling DateTimeOffset > DateTime conversion and vice versa for object types.
2. Making the value converter for the Date property editor more resilient.

Unit tests have been added to guard against this issue later on.

Testing this

1. Create a V13 site.
2. Create some content with:
   • A "date without time" and a "date with time" property at content root level.
   • A block list with elements containing both "date without time" and a "date with time" properties.
3. Create a frontend rendering that outputs the content values.
4. Upgrade the site to V14.1.1
5. Verify that the frontend rendering still works.
6. Verify that the back-office editing still works.

[Migaroez]

Frontend rendering is ok, but the backend seems to have lost it's time picker

[Migaroez]

Resaving data type applied correct time.