Fix getting of epic board parent

What does this MR do and why?

When checking feature flag in BoardItemFilterable module we need to get board's resoruce parent (project or group), but for epic boards resolvers we need to refer to use epic_board (instead of board).

Related to #382969 (comment 1271412420)

Screenshots or screen recordings

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How to set up and validate locally

1. Enable or filtering

   Feature.enable(:or_issuable_queries)

2. Sample query

query {
  group(fullPath: "flightjs") {
    id
    epicBoards {
      nodes {
        id
        lists(epicFilters: {or: {labelName: ["foo"]}}) {
          nodes {
            id
          }
        }
      }
    }
  }
}

MR acceptance checklist

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