Fix epochDelegateAccount GraphQL command to match its description

[mrmr1993]

This PR fixes epochDelegateAccount so that its behaviour matches the description, returning the account from the staking ledger to which this account has delegated. It has been broken since its inception in #3904, where its behaviour has been to always return the same account from the current staking ledger.

[mrmr1993]

!ci-build-me

[svv232]

I have tested this with a local devnet node running these changes:

this request:

query MyQuery {
  version
  account(publicKey: "B62qpfgnUm7zVqi8MJHNB2m37rtgMNDbFNhC2DpMmmVpQt8x6gKv9Ww") {
    epochDelegateAccount {
      nonce
      inferredNonce
      receiptChainHash
      delegate
      votingFor
      locked
      index
      zkappUri
      provedState
      tokenSymbol
      leafHash
    }
  }
}

yields this response:

{
  "data": {
    "version": "7bbbbff4ffc9c44f2ef55c88f24f302183e03080",
    "account": {
      "epochDelegateAccount": {
        "nonce": "2",
        "inferredNonce": "2",
        "receiptChainHash": "2n22gwywRfoiMv9hraotYwan7yRm78pwvdgDBcSfW5TYbtJVgp92",
        "delegate": "B62qohbCC7T1vuZcoRifiu7t2T9nWeWKmmB84XpzEMN2LnhEMw6Pkd5",
        "votingFor": "2mzWc7ZF8hRKvLyoLpKLZ3TSm8XSjfxZq67dphJpnjhYmU5Vja5Q",
        "locked": null,
        "index": 10747,
        "zkappUri": null,
        "provedState": null,
        "tokenSymbol": "",
        "leafHash": "17116368938849220799805337553686664893477144178746778846453787514071392809639"
      }
    }
  }
}

notice how the public key and the delegate address differ along with the nonce in accordance with the account information in https://minascan.io/devnet/account/B62qpfgnUm7zVqi8MJHNB2m37rtgMNDbFNhC2DpMmmVpQt8x6gKv9Ww/txs