Fetch child epics with colours when FF is enabled

What does this MR do and why?

When fetching Child Epics on the roadmap page, with the epic_color_highlight feature flag enabled, we need to fetch all of the child epics and also request the color and textColor attributes as part of the GraphQL query.

The top level epics make use of the groupEpics query, but child epics use a different query called epicChildEpics. In the previous MR (Use Epic colours for display on the roadmap (!89709 - merged)), this second query was missed and thus didn't get updated.

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How to set up and validate locally

1. Enable epic_color_highlight feature flag (Feature.enable(:epic_color_highlight, Group.find_by_full_path('flightjs')))
2. Update an epic in the flightjs group so that is has a parent epic and a colour other than Blue
3. Visit the roadmap page (http://gdk.test:3000/groups/flightjs/-/roadmap)
4. Expand the parent epic and notice that the colour of the child epic is always Blue
5. Apply this MR and reload the page
6. Expanding the child epic will now fetch the correct epic colour

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #7641 (closed)

ee/app/assets/javascripts/roadmap/queries/epic_child_epics_with_color.query.graphql

-#import "./epic.fragment.graphql"
-
-query epicChildEpicsWithColor(
-  $fullPath: ID!
-  $iid: ID!
-  $state: EpicState
-  $sort: EpicSort
-  $timeframe: Timeframe
-  $labelName: [String!] = []
-  $authorUsername: String = ""
-  $confidential: Boolean
-  $search: String = ""
-) {
-  group(fullPath: $fullPath) {
-    id
-    name
-    epic(iid: $iid) {
-      id
-      title
-      hasChildren
-      children(
-        state: $state
-        sort: $sort
-        labelName: $labelName
-        authorUsername: $authorUsername
-        confidential: $confidential
-        search: $search
-        timeframe: $timeframe
-      ) {
-        edges {
-          node {
-            ...BaseEpic
-            color
-            textColor
-          }
-        }
-      }
-    }
-  }
-}