multiprocess: Make interfaces::Chain::isTaprootActive non-const

interfaces::Chain is an abstract class, so declaring the method const
would be exposing internal implementation details of subclasses to
interface callers. And specifically this doesn't work because the
multiprocess implementation of the interfaces::Chain::isTaprootActive
method can't be const because IPC connection state and request state is
not constant during the call.

src/node/interfaces.cpp

@@ -698,7 +698,7 @@ public:
             notifications.transactionAddedToMempool(entry.GetSharedTx(), 0 /* mempool_sequence */);
         }
     }
-    bool isTaprootActive() const override
+    bool isTaprootActive() override
     {
         LOCK(::cs_main);
         const CBlockIndex* tip = Assert(m_node.chainman)->ActiveChain().Tip();