fix: createwallet to require 'load_on_startup' for descriptor wallets

createwallet has changed list of arguments: createwallet "wallet_name" ( disable_private_keys blank "passphrase" avoid_reuse descriptors load_on_startup )
load_on_startup used to be an argument 5 but now has a number 6.
Both arguments 5 and 6 are boolean and it can confuse an user.

To prevent confusion if user is not aware about this breaking changes,
the RPC createwallet throws an exception if user trying to create descriptor wallet but has not mentioned load_on_startup.
This requirement can be removed when major amount of users updated to v21

src/wallet/rpcwallet.cpp

@@ -3032,6 +3032,9 @@ static RPCHelpMan createwallet()
 #ifndef USE_SQLITE
         throw JSONRPCError(RPC_WALLET_ERROR, "Compiled without sqlite support (required for descriptor wallets)");
 #endif
+        if (request.params[6].isNull()) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "RPC createwallet would not accept creating descriptor wallets without specifying 'load_on_startup' flag. It is required explicitly in v21 due to breaking changes in createwallet RPC");
+        }
         flags |= WALLET_FLAG_DESCRIPTORS;
         warnings.emplace_back(Untranslated("Wallet is an experimental descriptor wallet"));
     }

test/functional/test_framework/test_node.py

@@ -704,6 +704,8 @@ def __getattr__(self, name):
     def createwallet(self, wallet_name, disable_private_keys=None, blank=None, passphrase='', avoid_reuse=None, descriptors=None, load_on_startup=None):
         if descriptors is None:
             descriptors = self.descriptors
+        if descriptors is not None and load_on_startup is None:
+            load_on_startup = False
         return self.__getattr__('createwallet')(wallet_name, disable_private_keys, blank, passphrase, avoid_reuse, descriptors, load_on_startup)
 
     def importprivkey(self, privkey, label=None, rescan=None):