RPCs that return detailed information about mempool entries (e.g. `getmempoolentry` and `getrawmempool(verbose=True)` include the fields `fee`, `modifiedfee`, `descendantfees`, and `ancestorfees` which were marked as "DEPRECATED" and replaced with a `fees` object field in #12240 (since v0.17).

We should remove these fields, but first, properly deprecate them using `-deprecatedrpc`.

#### Useful skills:

- basic understanding of Bitcoin Core RPC interface

#### Want to work on this issue?

For guidance on contributing, please read [CONTRIBUTING.md](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md) before opening your pull request.

if (pcursor->GetKey(key) && pcursor->GetValue(coin)) {
            afile << key;
            afile << coin;
        }
        pcursor->Next();
    }
    afile.fclose();
    UniValue result(UniValue::VOBJ);
    result.pushKV("coins_written", stats.coins_count);
    result.pushKV("base_hash", tip->GetBlockHash().ToString());
    result.pushKV("base_height", tip->nHeight);
    result.pushKV("path", path.u8string());
    result.pushKV("txoutset_hash", stats.hashSerialized.ToString());
    
    
    result.pushKV("nchaintx", uint64_t{tip->nChainTx});
    return result;
}
void RegisterBlockchainRPCCommands(CRPCTable &t)
{
static const CRPCCommand commands[] =
{ 
  
    { "blockchain",         &getblockchaininfo,                  },
    { "blockchain",         &getchaintxstats,                    },
    { "blockchain",         &getblockstats,                      },
    { "blockchain",         &getbestblockhash,                   },
    { "blockchain",         &getblockcount,                      },
    { "blockchain",         &getblock,                           },
    { "blockchain",         &getblockhash,                       },