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.

},
                {
                    RPCResult{"for verbosity = 0",
                RPCResult::Type::STR_HEX, "", "A string that is serialized, hex-encoded data for block 'hash'"},
                    RPCResult{"for verbosity = 1",
                RPCResult::Type::OBJ, "", "",
                {
                    {RPCResult::Type::STR_HEX, "hash", "the block hash (same as provided)"},
                    {RPCResult::Type::NUM, "confirmations", "The number of confirmations, or -1 if the block is not on the main chain"},
                    {RPCResult::Type::NUM, "size", "The block size"},
                    {RPCResult::Type::NUM, "strippedsize", "The block size excluding witness data"},
                    {RPCResult::Type::NUM, "weight", "The block weight as defined in BIP 141"},
                    {RPCResult::Type::NUM, "height", "The block height or index"},
                    {RPCResult::Type::NUM, "version", "The block version"},
                    {RPCResult::Type::STR_HEX, "versionHex", "The block version formatted in hexadecimal"},
                    {RPCResult::Type::STR_HEX, "merkleroot", "The merkle root"},
                    {RPCResult::Type::ARR, "tx", "The transaction ids",
                        {{RPCResult::Type::STR_HEX, "", "The transaction id"}}},
                    {RPCResult::Type::NUM_TIME, "time",       "The block time expressed in " + UNIX_EPOCH_TIME},
                    {RPCResult::Type::NUM_TIME, "mediantime", "The median block time expressed in " + UNIX_EPOCH_TIME},
                    {RPCResult::Type::NUM, "nonce", "The nonce"},
                    {RPCResult::Type::STR_HEX, "bits", "The bits"},
                    {RPCResult::Type::NUM, "difficulty", "The difficulty"},
                    {RPCResult::Type::STR_HEX, "chainwork", "Expected number of hashes required to produce the chain up to this block (in hex)"},
                    {RPCResult::Type::NUM, "nTx", "The number of transactions in the block"},
                    {RPCResult::Type::STR_HEX, "previousblockhash",  true, "The hash of the previous block (if available)"},
                    {RPCResult::Type::STR_HEX, "nextblockhash",  true, "The hash of the next block (if available)"},
                }},
                    RPCResult{"for verbosity = 2",
                RPCResult::Type::OBJ, "", "",