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.

{
    
    LOCK(pool.cs);
    UniValue ret(UniValue::VOBJ);
    ret.pushKV("loaded", pool.IsLoaded());
    ret.pushKV("size", (int64_t)pool.size());
    ret.pushKV("bytes", (int64_t)pool.GetTotalTxSize());
    ret.pushKV("usage", (int64_t)pool.DynamicMemoryUsage());
    ret.pushKV("total_fee", ValueFromAmount(pool.GetTotalFee()));
    size_t maxmempool = gArgs.GetIntArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000;
    ret.pushKV("maxmempool", (int64_t) maxmempool);
    ret.pushKV("mempoolminfee", ValueFromAmount(std::max(pool.GetMinFee(maxmempool), ::minRelayTxFee).GetFeePerK()));
    ret.pushKV("minrelaytxfee", ValueFromAmount(::minRelayTxFee.GetFeePerK()));
    ret.pushKV("unbroadcastcount", uint64_t{pool.GetUnbroadcastTxs().size()});
    return ret;
}
static RPCHelpMan getmempoolinfo()