Output culture variant update dates from the Delivery API

[kjac]

Prerequisites

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

Description

The Delivery API current outputs the "global" update date for culture variant content, which makes it hard to detect culture specific changes.

This PR updates the Delivery API output, so the updateDate property reflects the "culture date" of the requested content.

Testing this PR

Verify that:

• updateDate reflects the "culture date" for culture variant content.
  • More info on how these are supposed to work and when they should update can be found in the description of Make culture specific update dates work again #19145
• updateDate still works for invariant content.

[AndyButland]

Code and tests all look good to me.

Tested with the same setup as the linked PR - with this data output in Razor:

Published culture date (en): 4/29/2025 4:27:12 AM
Published culture date (it): 4/25/2025 2:23:13 PM

Request to retrieve item with en-US language:

{
    "contentType": "testPage",
    "name": "Test Page",
    "createDate": "2025-04-23T12:09:24.693Z",
    "updateDate": "2025-04-29T04:27:12.2159775Z",
    ...

Request to retrieve item with it language:

{
    "contentType": "testPage",
    "name": "Test Page",
    "createDate": "2025-04-23T12:09:24.693Z",
    "updateDate": "2025-04-25T14:23:13.267Z",
    ...

Request for another invariant page also returns the expected value.