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

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{
    return EnsureMemPool(EnsureAnyNodeContext(context));
}
ChainstateManager& EnsureChainman(const NodeContext& node)
{
    if (!node.chainman) {
        throw JSONRPCError(RPC_INTERNAL_ERROR, "Node chainman not found");
    }
    return *node.chainman;
}
ChainstateManager& EnsureAnyChainman(const std::any& context)
{
    return EnsureChainman(EnsureAnyNodeContext(context));
}
CBlockPolicyEstimator& EnsureFeeEstimator(const NodeContext& node)
{
    if (!node.fee_estimator) {
        throw JSONRPCError(RPC_INTERNAL_ERROR, "Fee estimation disabled");
    }
    return *node.fee_estimator;
}
CBlockPolicyEstimator& EnsureAnyFeeEstimator(const std::any& context)
{
    return EnsureFeeEstimator(EnsureAnyNodeContext(context));
}
/* Calculate the difficulty for a given block index.
 */
double GetDifficulty(const CBlockIndex* blockindex)