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.

},
                        "stats"},
                },
                RPCResult{
            RPCResult::Type::OBJ, "", "",
            {
                {RPCResult::Type::NUM, "avgfee",  true, "Average fee in the block"},
                {RPCResult::Type::NUM, "avgfeerate",  true, "Average feerate (in satoshis per virtual byte)"},
                {RPCResult::Type::NUM, "avgtxsize",  true, "Average transaction size"},
                {RPCResult::Type::STR_HEX, "blockhash",  true, "The block hash (to check for potential reorgs)"},
                {RPCResult::Type::ARR_FIXED, "feerate_percentiles",  true, "Feerates at the 10th, 25th, 50th, 75th, and 90th percentile weight unit (in satoshis per virtual byte)",
                {
                    {RPCResult::Type::NUM, "10th_percentile_feerate", "The 10th percentile feerate"},
                    {RPCResult::Type::NUM, "25th_percentile_feerate", "The 25th percentile feerate"},
                    {RPCResult::Type::NUM, "50th_percentile_feerate", "The 50th percentile feerate"},
                    {RPCResult::Type::NUM, "75th_percentile_feerate", "The 75th percentile feerate"},
                    {RPCResult::Type::NUM, "90th_percentile_feerate", "The 90th percentile feerate"},
                }},
                {RPCResult::Type::NUM, "height",  true, "The height of the block"},
                {RPCResult::Type::NUM, "ins",  true, "The number of inputs (excluding coinbase)"},
                {RPCResult::Type::NUM, "maxfee",  true, "Maximum fee in the block"},
                {RPCResult::Type::NUM, "maxfeerate",  true, "Maximum feerate (in satoshis per virtual byte)"},
                {RPCResult::Type::NUM, "maxtxsize",  true, "Maximum transaction size"},
                {RPCResult::Type::NUM, "medianfee",  true, "Truncated median fee in the block"},
                {RPCResult::Type::NUM, "mediantime",  true, "The block median time past"},
                {RPCResult::Type::NUM, "mediantxsize",  true, "Truncated median transaction size"},
                {RPCResult::Type::NUM, "minfee",  true, "Minimum fee in the block"},
                {RPCResult::Type::NUM, "minfeerate",  true, "Minimum feerate (in satoshis per virtual byte)"},
                {RPCResult::Type::NUM, "mintxsize",  true, "Minimum transaction size"},
                {RPCResult::Type::NUM, "outs",  true, "The number of outputs"},