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

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ret_all.pushKV("swtotal_size", swtotal_size);
    ret_all.pushKV("swtotal_weight", swtotal_weight);
    ret_all.pushKV("swtxs", swtxs);
    ret_all.pushKV("time", pindex->GetBlockTime());
    ret_all.pushKV("total_out", total_out);
    ret_all.pushKV("total_size", total_size);
    ret_all.pushKV("total_weight", total_weight);
    ret_all.pushKV("totalfee", totalfee);
    ret_all.pushKV("txs", (int64_t)block.vtx.size());
    ret_all.pushKV("utxo_increase", outputs - inputs);
    ret_all.pushKV("utxo_size_inc", utxo_size_inc);
    if (do_all) {
        return ret_all;
    }
    UniValue ret(UniValue::VOBJ);
    for (const std::string& stat : stats) {
        const UniValue& value = ret_all[stat];
        if (value.isNull()) {
            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid selected statistic %s", stat));
        }
        ret.pushKV(stat, value);
    }
    return ret;
},
    };
}
static RPCHelpMan savemempool()
{
    return RPCHelpMan{"savemempool",
                "\nDumps the mempool to disk. It will fail until the previous dump is fully loaded.\n",