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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CChain& active_chain = chainman.ActiveChain();
    /*
     * Idea: The set of chain tips is the active chain tip, plus orphan blocks which do not have another orphan building off of them.
     * Algorithm:
     *  - Make one pass through BlockIndex(), picking out the orphan blocks, and also storing a set of the orphan block's pprev pointers.
     *  - Iterate through the orphan blocks. If the block isn't pointed to by another orphan, it is a chain tip.
     *  - Add the active chain tip
     */
    std::set<const CBlockIndex*, CompareBlocksByHeight> setTips;
    std::set<const CBlockIndex*> setOrphans;
    std::set<const CBlockIndex*> setPrevs;
    for (const std::pair<const uint256, CBlockIndex*>& item : chainman.BlockIndex()) {
        if (!active_chain.Contains(item.second)) {
            setOrphans.insert(item.second);
            setPrevs.insert(item.second->pprev);
        }
    }
    for (std::set<const CBlockIndex*>::iterator it = setOrphans.begin(); it != setOrphans.end(); ++it) {
        if (setPrevs.erase(*it) == 0) {
            setTips.insert(*it);
        }
    }
    
    setTips.insert(active_chain.Tip());
    
    UniValue res(UniValue::VARR);
    for (const CBlockIndex* block : setTips) {
        UniValue obj(UniValue::VOBJ);
        obj.pushKV("height", block->nHeight);
        obj.pushKV("hash", block->phashBlock->GetHex());