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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For guidance on contributing, please read [CONTRIBUTING.md](https://github.com/bitcoin/bitcoin/blob/master/CONTRIBUTING.md) before opening your pull request.

{
    auto node_context = util::AnyPtr<NodeContext>(context);
    if (!node_context) {
        throw JSONRPCError(RPC_INTERNAL_ERROR, "Node context not found");
    }
    return *node_context;
}
CTxMemPool& EnsureMemPool(const NodeContext& node)
{
    if (!node.mempool) {
        throw JSONRPCError(RPC_CLIENT_MEMPOOL_DISABLED, "Mempool disabled or instance not found");
    }
    return *node.mempool;
}
CTxMemPool& EnsureAnyMemPool(const std::any& context)