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.

NodeContext& node,
    CChainState& chainstate,
    CAutoFile& afile,
    const fs::path& path,
    const fs::path& temppath)
{
    std::unique_ptr<CCoinsViewCursor> pcursor;
    CCoinsStats stats{CoinStatsHashType::HASH_SERIALIZED};
    CBlockIndex* tip;
    {