Fix work items fixed dates inconsistency

What does this MR do and why?

Related to #477496 (closed)

To match the behaviour seen in legacy epics, the attributes dates_source.start_date and dates_source.due_date should be updated to match the respective *_date_fixed field when dates are changes to be fixed.

MR acceptance checklist

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✅

How to set up and validate locally

1. In rails console enable the feature flags

   Feature.enable(:work_item_epics)
   Feature.enable(:work_item_epics_rollout)
   Feature.enable(:work_items_rolledup_dates)

2. Visit a group and create a new epic with fixed dates
3. Open the new open and change the dates to Inherited. The dates will now be None because the epic has no children.
4. Change the dates back to Fixed, the dates should change back to the original value set during creation
5. Visit http://127.0.0.1:3000/-/graphql-explorer and use the following queries to verify that the values start_date and due_date are set correctly for the legacy epic and the synced work item:

work item dates query

query getWorkItemInSync {
  workspace: group(fullPath: "<GROUP_PATH>") {
    workItems(iid: "<EPIC_IID>") {
      edges {
        node {
          widgets {
            ... on WorkItemWidgetRolledupDates {
              startDate
              startDateFixed
              startDateIsFixed
              dueDate
              dueDateFixed
              dueDateIsFixed
            }
            ... on WorkItemWidgetStartAndDueDate {
              startDate
              dueDate
            }
          }
        }
      }
    }
  }
}

response

{
  "data": {
    "workspace": {
      "workItems": {
        "edges": [
          {
            "node": {
              "widgets": [
                {
                  "startDate": "2024-08-01",
                  "startDateFixed": "2024-08-01",
                  "startDateIsFixed": true,
                  "startDateSourcingMilestone": null,
                  "dueDate": "2024-08-31",
                  "dueDateFixed": "2024-08-31",
                  "dueDateIsFixed": true,
                  "dueDateSourcingMilestone": null
                },
                {
                  "startDate": "2024-08-01",
                  "dueDate": "2024-08-31"
                }
              ]
            }
          }
        ]
      }
    }
  }
}

work item dates query

query getEpicInSync {
  group(fullPath: "<GROUP_PATH>") {
    epics(iid: "<EPIC_IID>") {
      edges {
        node {
          startDate
          startDateFixed
          startDateIsFixed
          dueDate
          dueDateFixed
          dueDateIsFixed
        }
      }
    }
  }
}

response

{
  "data": {
    "group": {
      "epics": {
        "edges": [
          {
            "node": {
              "startDate": "2024-08-01",
              "startDateFixed": "2024-08-01",
              "startDateIsFixed": true,
              "startDateFromInheritedSource": null,
              "dueDate": "2024-08-31",
              "dueDateFixed": "2024-08-31",
              "dueDateIsFixed": true,
              "dueDateFromInheritedSource": null
            }
          }
        ]
      }
    }
  }
}