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.

#include <rpc/blockchain.h>
#include <blockfilter.h>
#include <chain.h>
#include <chainparams.h>
#include <coins.h>
#include <consensus/amount.h>
#include <consensus/params.h>
#include <consensus/validation.h>
#include <core_io.h>
#include <deploymentinfo.h>
#include <deploymentstatus.h>
#include <fs.h>
#include <hash.h>
#include <index/blockfilterindex.h>
#include <index/coinstatsindex.h>
#include <node/blockstorage.h>
#include <logging/timer.h>
#include <node/coinstats.h>
#include <node/context.h>
#include <node/utxo_snapshot.h>
#include <policy/feerate.h>
#include <policy/fees.h>
#include <policy/policy.h>
#include <policy/rbf.h>
#include <primitives/transaction.h>