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.

{
    return RPCHelpMan{"getmempooldescendants",
                "\nIf txid is in the mempool, returns all in-mempool descendants.\n",
                {
                    {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id (must be in mempool)"},
                    {"verbose", RPCArg::Type::BOOL, RPCArg::Default{false}, "True for a json object, false for array of transaction ids"},
                },
                {
                    RPCResult{"for verbose = false",
                        RPCResult::Type::ARR, "", "",
                        {{RPCResult::Type::STR_HEX, "", "The transaction id of an in-mempool descendant transaction"}}},
                    RPCResult{"for verbose = true",
                        RPCResult::Type::OBJ_DYN, "", "",
                        {
                            {RPCResult::Type::OBJ, "transactionid", "", MempoolEntryDescription()},
                        }},
                },
                RPCExamples{
                    HelpExampleCli("getmempooldescendants", "\"mytxid\"")
            + HelpExampleRpc("getmempooldescendants", "\"mytxid\"")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    bool fVerbose = false;
    if (!request.params[1].isNull())
        fVerbose = request.params[1].get_bool();
    uint256 hash = ParseHashV(request.params[0], "parameter 1");
    const CTxMemPool& mempool = EnsureAnyMemPool(request.context);