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{"getblockchaininfo",
                "Returns an object containing various state info regarding blockchain processing.\n",
                {},
                RPCResult{
                    RPCResult::Type::OBJ, "", "",
                    {
                        {RPCResult::Type::STR, "chain", "current network name (main, test, signet, regtest)"},
                        {RPCResult::Type::NUM, "blocks", "the height of the most-work fully-validated chain. The genesis block has height 0"},
                        {RPCResult::Type::NUM, "headers", "the current number of headers we have validated"},
                        {RPCResult::Type::STR, "bestblockhash", "the hash of the currently best block"},
                        {RPCResult::Type::NUM, "difficulty", "the current difficulty"},
                        {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, "verificationprogress", "estimate of verification progress [0..1]"},
                        {RPCResult::Type::BOOL, "initialblockdownload", "(debug information) estimate of whether this node is in Initial Block Download mode"},
                        {RPCResult::Type::STR_HEX, "chainwork", "total amount of work in active chain, in hexadecimal"},
                        {RPCResult::Type::NUM, "size_on_disk", "the estimated size of the block and undo files on disk"},
                        {RPCResult::Type::BOOL, "pruned", "if the blocks are subject to pruning"},
                        {RPCResult::Type::NUM, "pruneheight",  true, "lowest-height complete block stored (only present if pruning is enabled)"},
                        {RPCResult::Type::BOOL, "automatic_pruning",  true, "whether automatic pruning is enabled (only present if pruning is enabled)"},
                        {RPCResult::Type::NUM, "prune_target_size",  true, "the target size used by pruning (only present if automatic pruning is enabled)"},
                        {RPCResult::Type::OBJ_DYN, "softforks", "status of softforks",
                        {
                            {RPCResult::Type::OBJ, "xxxx", "name of the softfork",
                            {