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.

ChainstateManager& chainman = EnsureAnyChainman(request.context);
    LOCK(cs_main);
    CChainState& active_chainstate = chainman.ActiveChainstate();
    const CBlockIndex* tip = active_chainstate.m_chain.Tip();
    CHECK_NONFATAL(tip);
    const int height = tip->nHeight;
    UniValue obj(UniValue::VOBJ);
    obj.pushKV("chain",                 Params().NetworkIDString());
    obj.pushKV("blocks",                height);
    obj.pushKV("headers",               pindexBestHeader ? pindexBestHeader->nHeight : -1);
    obj.pushKV("bestblockhash",         tip->GetBlockHash().GetHex());
    obj.pushKV("difficulty",            (double)GetDifficulty(tip));
    obj.pushKV("time",                  (int64_t)tip->nTime);
    obj.pushKV("mediantime",            (int64_t)tip->GetMedianTimePast());
    obj.pushKV("verificationprogress",  GuessVerificationProgress(Params().TxData(), tip));
    obj.pushKV("initialblockdownload",  active_chainstate.IsInitialBlockDownload());
    obj.pushKV("chainwork",             tip->nChainWork.GetHex());
    obj.pushKV("size_on_disk",          CalculateCurrentUsage());
    obj.pushKV("pruned",                fPruneMode);
    if (fPruneMode) {
        const CBlockIndex* block = tip;
        CHECK_NONFATAL(block);
        while (block->pprev && (block->pprev->nStatus & BLOCK_HAVE_DATA)) {
            block = block->pprev;
        }
        obj.pushKV("pruneheight",        block->nHeight);
        
        bool automatic_pruning = (gArgs.GetIntArg("-prune", 0) != 1);
        obj.pushKV("automatic_pruning",  automatic_pruning);
        if (automatic_pruning) {