The wallet code uses `Optional::emplace()`, which was only added in boost 1.56, see https://github.com/boostorg/optional/commit/2e583aaf30964afd66c1284aa71d0d8b42fa440d

So for 0.21.0 there are two options:
* Work around `emplace` and keep the minimum boost version below 1.56
* Require boost 1.56


```
wallet/rpcwallet.cpp: In function ‘UniValue listsinceblock(const JSONRPCRequest&)’:
wallet/rpcwallet.cpp:1560:16: error: ‘Optional<int>’ has no member named ‘emplace’
         height.emplace();
                ^
wallet/rpcwallet.cpp:1561:19: error: ‘Optional<int>’ has no member named ‘emplace’
         altheight.emplace();
                   ^
{
            RPCHelpMan{"loadwallet",
                "\nLoads a wallet from a wallet file or directory."
                "\nNote that all wallet command-line options used when starting bitcoind will be"
                "\napplied to the new wallet (eg -zapwallettxes, rescan, etc).\n",
                {
                    {"filename", RPCArg::Type::STR, RPCArg::Optional::NO, "The wallet directory or .dat file."},
                },
                RPCResult{
                    RPCResult::Type::OBJ, "", "",