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.

{
    CHECK_NONFATAL(blockindex);
    int nShift = (blockindex->nBits >> 24) & 0xff;
    double dDiff =
        (double)0x0000ffff / (double)(blockindex->nBits & 0x00ffffff);
    while (nShift < 29)
    {
        dDiff *= 256.0;
        nShift++;
    }
    while (nShift > 29)
    {
        dDiff /= 256.0;
        nShift--;
    }
    return dDiff;
}
static int ComputeNextBlockAndDepth(const CBlockIndex* tip, const CBlockIndex* blockindex, const CBlockIndex*& next)
{
    next = tip->GetAncestor(blockindex->nHeight + 1);
    if (next && next->pprev == blockindex) {
        return tip->nHeight - blockindex->nHeight + 1;
    }
    next = nullptr;
    return blockindex == tip ? 1 : -1;
}
CBlockIndex* ParseHashOrHeight(const UniValue& param, ChainstateManager& chainman) {