rpc: Select int-UniValue constructor for enum value in upgradewallet RPC

UniValue does not have a constructor for enum values, however the
compiler will decay the enum into an int and select that constructor.
Avoid this compiler magic and clarify the code by explicitly selecting
the int-constructor.

This is needed for the next commit.

Author: MacroFake

src/wallet/rpc/wallet.cpp

@@ -532,7 +532,7 @@ static RPCHelpMan upgradewallet()
         "\nUpgrade the wallet. Upgrades to the latest version if no version number is specified.\n"
         "New keys may be generated and a new wallet backup will need to be made.",
         {
-            {"version", RPCArg::Type::NUM, RPCArg::Default{FEATURE_LATEST}, "The version number to upgrade to. Default is the latest wallet version."}
+            {"version", RPCArg::Type::NUM, RPCArg::Default{int{FEATURE_LATEST}}, "The version number to upgrade to. Default is the latest wallet version."}
         },
         RPCResult{
             RPCResult::Type::OBJ, "", "",