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.

+ HelpExampleRpc("preciousblock", "\"blockhash\"")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    uint256 hash(ParseHashV(request.params[0], "blockhash"));
    CBlockIndex* pblockindex;
    ChainstateManager& chainman = EnsureAnyChainman(request.context);
    {
        LOCK(cs_main);
        pblockindex = chainman.m_blockman.LookupBlockIndex(hash);
        if (!pblockindex) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        }
    }
    BlockValidationState state;
    chainman.ActiveChainstate().PreciousBlock(state, pblockindex);
    if (!state.IsValid()) {
        throw JSONRPCError(RPC_DATABASE_ERROR, state.ToString());
    }
    return NullUniValue;
},
    };
}
static RPCHelpMan invalidateblock()
{
    return RPCHelpMan{"invalidateblock",
                "\nPermanently marks a block as invalid, as if it violated a consensus rule.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "the hash of the block to mark as invalid"},
                },