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.

{
    return RPCHelpMan{"getblockheader",
                "\nIf verbose is false, returns a string that is serialized, hex-encoded data for blockheader 'hash'.\n"
                "If verbose is true, returns an Object with information about blockheader <hash>.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The block hash"},
                    {"verbose", RPCArg::Type::BOOL, RPCArg::Default{true}, "true for a json object, false for the hex-encoded data"},
                },
                {
                    RPCResult{"for verbose = true",
                        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, "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"},