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();
                   ^
{
    if (pwallet->IsLocked()) {
        throw JSONRPCError(RPC_WALLET_UNLOCK_NEEDED, "Error: Please enter the wallet passphrase with walletpassphrase first.");
    }
}
LegacyScriptPubKeyMan& EnsureLegacyScriptPubKeyMan(CWallet& wallet, bool also_create)
{
    LegacyScriptPubKeyMan* spk_man = wallet.GetLegacyScriptPubKeyMan();
    if (!spk_man && also_create) {
        spk_man = wallet.GetOrCreateLegacyScriptPubKeyMan();
    }
    if (!spk_man) {
        throw JSONRPCError(RPC_WALLET_ERROR, "This type of wallet does not support this command");
    }
    return *spk_man;
}
static void WalletTxToJSON(interfaces::Chain& chain, const CWalletTx& wtx, UniValue& entry)
{
    int confirms = wtx.GetDepthInMainChain();
    entry.pushKV("confirmations", confirms);
    if (wtx.IsCoinBase())
        entry.pushKV("generated", true);
    if (confirms > 0)
    {
        entry.pushKV("blockhash", wtx.m_confirm.hashBlock.GetHex());