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.

const int branchLen = block->nHeight - active_chain.FindFork(block)->nHeight;
        obj.pushKV("branchlen", branchLen);
        std::string status;
        if (active_chain.Contains(block)) {
            
            status = "active";
        } else if (block->nStatus & BLOCK_FAILED_MASK) {
            
            status = "invalid";
        } else if (!block->HaveTxsDownloaded()) {
            
            status = "headers-only";
        } else if (block->IsValid(BLOCK_VALID_SCRIPTS)) {
            
            status = "valid-fork";
        } else if (block->IsValid(BLOCK_VALID_TREE)) {
            
            status = "valid-headers";
        } else {
            
            status = "unknown";
        }
        obj.pushKV("status", status);
        res.push_back(obj);
    }
    return res;
},
    };
}
UniValue MempoolInfoToJSON(const CTxMemPool& pool)