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

g_should_abort_scan = false;
        int64_t count = 0;
        std::unique_ptr<CCoinsViewCursor> pcursor;
        CBlockIndex* tip;
        NodeContext& node = EnsureAnyNodeContext(request.context);
        {
            ChainstateManager& chainman = EnsureChainman(node);
            LOCK(cs_main);
            CChainState& active_chainstate = chainman.ActiveChainstate();
            active_chainstate.ForceFlushStateToDisk();
            pcursor = active_chainstate.CoinsDB().Cursor();
            CHECK_NONFATAL(pcursor);
            tip = active_chainstate.m_chain.Tip();
            CHECK_NONFATAL(tip);
        }
        bool res = FindScriptPubKey(g_scan_progress, g_should_abort_scan, count, pcursor.get(), needles, coins, node.rpc_interruption_point);
        result.pushKV("success", res);
        result.pushKV("txouts", count);
        result.pushKV("height", tip->nHeight);
        result.pushKV("bestblock", tip->GetBlockHash().GetHex());
        for (const auto& it : coins) {
            const COutPoint& outpoint = it.first;
            const Coin& coin = it.second;
            const CTxOut& txo = coin.out;
            input_txos.push_back(txo);
            total_in += txo.nValue;
            UniValue unspent(UniValue::VOBJ);
            unspent.pushKV("txid", outpoint.hash.GetHex());
            unspent.pushKV("vout", (int32_t)outpoint.n);
            unspent.pushKV("scriptPubKey", HexStr(txo.scriptPubKey));