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::Type::STR, "type", "one of \"buried\", \"bip9\""},
                                {RPCResult::Type::OBJ, "bip9",  true, "status of bip9 softforks (only for \"bip9\" type)",
                                {
                                    {RPCResult::Type::STR, "status", "one of \"defined\", \"started\", \"locked_in\", \"active\", \"failed\""},
                                    {RPCResult::Type::NUM, "bit",  true, "the bit (0-28) in the block version field used to signal this softfork (only for \"started\" and \"locked_in\" status)"},
                                    {RPCResult::Type::NUM_TIME, "start_time", "the minimum median time past of a block at which the bit gains its meaning"},
                                    {RPCResult::Type::NUM_TIME, "timeout", "the median time past of a block at which the deployment is considered failed if not yet locked in"},
                                    {RPCResult::Type::NUM, "since", "height of the first block to which the status applies"},
                                    {RPCResult::Type::NUM, "min_activation_height", "minimum height of blocks for which the rules may be enforced"},
                                    {RPCResult::Type::OBJ, "statistics",  true, "numeric statistics about signalling for a softfork (only for \"started\" and \"locked_in\" status)",
                                    {
                                        {RPCResult::Type::NUM, "period", "the length in blocks of the signalling period"},
                                        {RPCResult::Type::NUM, "threshold",  true, "the number of blocks with the version bit set required to activate the feature (only for \"started\" status)"},
                                        {RPCResult::Type::NUM, "elapsed", "the number of blocks elapsed since the beginning of the current period"},
                                        {RPCResult::Type::NUM, "count", "the number of blocks with the version bit set in the current period"},
                                        {RPCResult::Type::BOOL, "possible",  true, "returns false if there are not enough blocks left in this period to pass activation threshold (only for \"started\" status)"},
                                    }},
                                }},
                                {RPCResult::Type::NUM, "height",  true, "height of the first block which the rules are or will be enforced (only for \"buried\" type, or \"bip9\" type with \"active\" status)"},
                                {RPCResult::Type::BOOL, "active", "true if the rules are enforced for the mempool and the next block"},
                            }},
                        }},
                        {RPCResult::Type::STR, "warnings", "any network and blockchain warnings"},
                    }},
                RPCExamples{
                    HelpExampleCli("getblockchaininfo", "")
            + HelpExampleRpc("getblockchaininfo", "")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{