Restrict submitpackage RPC: require Core 28.0.0

Apparently Core 27.0.0 has it but it's largely not useful. According to
ElectrumX devs. See: https://github.com/spesmilo/electrum-protocol/pull/6/files#r2459860616

Author: cculianu

src/BitcoinD.cpp

@@ -312,9 +312,10 @@ void BitcoinDMgr::refreshBitcoinDNetworkInfo()
                 // Bitcoin Core 25.0+ requires specifying `maxburnamount` in the `sendrawtransaction` RPC
                 // (which, due to bitcoin core weirdness in how it encodes the version int, maps to Version{0, 25, 0}
                 rsi.sendRawTransactionRequiresMaxBurnAmount = res.isCore && bitcoinDInfo.version >= Version{0, 25, 0};
-                // The `submitpackage` RPC is only present in a form usable by us on Bitcoin Core >= 27.0.0
-                rsi.hasSubmitPackageRPC = res.isCore && bitcoinDInfo.version >= Version{0, 27, 0};
-                rsi.submitPackageRPCSupportsMaxBurnAmount = rsi.hasSubmitPackageRPC && bitcoinDInfo.version >= Version{0, 28, 0};
+                // The `submitpackage` RPC is only present in a form usable by us on Bitcoin Core >= 27.0.0, but it was
+                // almost worthless on 27.0.0 (according to ElectrumX devs), and so we require 28.0.0.
+                // See: https://github.com/spesmilo/electrum-protocol/pull/6/files#r2459860616
+                rsi.hasSubmitPackageRPC = res.isCore && bitcoinDInfo.version >= Version{0, 28, 0};
             } // end lock scope
             // be sure to announce whether remote bitcoind is bitcoin core (this determines whether we use segwit or not)
             BTC::Coin coin = BTC::Coin::BCH; // default BCH if unknown (not segwit)
@@ -1070,7 +1071,6 @@ QVariantMap BitcoinDInfo::toVariantMap() const
     ret["hasDSProofRPC"] = rpcSupportInfo.hasDSProofRPC;
     ret["sendRawTransactionRequiresMaxBurnAmount"] = rpcSupportInfo.sendRawTransactionRequiresMaxBurnAmount;
     ret["hasSubmitPackageRPC"] = rpcSupportInfo.hasSubmitPackageRPC;
-    ret["submitPackageRPCSupportsMaxBurnAmount"] = rpcSupportInfo.submitPackageRPCSupportsMaxBurnAmount;
     ret["isCore"] = isCore;
     ret["isLTC"] = isLTC;
     ret["isBU"] = isBU;

src/BitcoinD.h

@@ -56,8 +56,7 @@ struct BitcoinDInfo {
         bool lacksGetZmqNotifications = false; ///< true if bchd or BU < 1.9.1.0, or if we got an RPC error the last time we queried
         bool hasDSProofRPC = false; ///< true if the RPC query to `getdsprooflist` didn't return an error.
         bool sendRawTransactionRequiresMaxBurnAmount = false; ///< true if the `sendrawtransaction` RPC requires 2 extra args. Bitcoin Core >= 25.0.0 only.
-        bool hasSubmitPackageRPC = false; ///< true if the `submitpackage` RPC method exists and is what we expect. Bitcoin Core >= 27.0.0 only.
-        bool submitPackageRPCSupportsMaxBurnAmount = false; ///< true if hasSubmitPackageRPC and Bitcoin Core >= 28.0.0.
+        bool hasSubmitPackageRPC = false; ///< true if the `submitpackage` RPC method exists and is what we expect. Bitcoin Core >= 28.0.0 only.
     };
     RpcSupportInfo rpcSupportInfo;
     bool isCore = false; ///< true if we are actually connected to /Satoshi.. node (Bitcoin Core)