Add color and health status GraphQL arguments for creating a work item

What does this MR do and why?

Related to #456119

Add support for setting health status and color when creating a work item. These arguments are needed for !144705 (closed) (see !144705 (comment 1861625817) for more context).

Summary of changes:

• Health status widget: We were using a service to update this widget and didn't have a create service so instead of creating it I took the opportunity to move the update logic to a callback class so it can be reused when creating the work item (this work was planned as part of #412801). Also, delete WorkItems::Widgets::HealthStatusService::UpdateService file and the corresponding specs, as well as removing the file name from the rubocop_todo lists.
• Color widget: Add argument to the create mutation, we already added an after_initialize callback for setting the value of this widget.

MR acceptance checklist

✅

Screenshots or screen recordings

mutation	result

How to set up and validate locally

1. Enable feature flag namespace_level_work_items
2. Visit https://gdk.test:3000/-/graphql-explorer and test the following query using the corresponding ID for workItemTypeId:

query

query getWorkItemTypeId{
  workspace: group(fullPath: "group-a") {
    workItemTypes(name: EPIC) {
      edges {
        node {
          id #=> E.g. "gid://gitlab/WorkItems::Type/8" but can be different in your local environment 
        }
      }
    }
  }
}

mutation createWIEpic {
  workItemCreate(input: {
    namespacePath: "group-a", title: "Epic WI with color and health status",
    workItemTypeId: "gid://gitlab/WorkItems::Type/8",
    colorWidget: {color: "#CCFFCC"},
    healthStatusWidget: { healthStatus: atRisk }
  }) {
    errors
    workItem {
      id
      webUrl
      widgets {
        ... on WorkItemWidgetHealthStatus {
          healthStatus
        }
        ... on WorkItemWidgetColor {
          color 
        }
      }
    }
  }
}

response

{
  "data": {
    "workItemCreate": {
      "errors": [],
      "workItem": {
        "id": "gid://gitlab/WorkItem/602",
        "webUrl": "http://127.0.0.1:3000/groups/group-a/-/work_items/138",
        "widgets": [
          {
            "healthStatus": "atRisk"
          },
          {
            "color": "#CCFFCC"
          }
        ]
      }
    }
  }
}

4. Visit the work item list using http://gdk.test:3000/groups/GROUP_PATH/-/work_items and verify that the the work item was created with the set arguments