rpc: call IsInitialBlockDownload via miner interface

src/rpc/mining.cpp

@@ -735,7 +735,7 @@ static RPCHelpMan getblocktemplate()
             throw JSONRPCError(RPC_CLIENT_NOT_CONNECTED, PACKAGE_NAME " is not connected!");
         }
 
-        if (chainman.IsInitialBlockDownload()) {
+        if (miner.isInitialBlockDownload()) {
             throw JSONRPCError(RPC_CLIENT_IN_INITIAL_DOWNLOAD, PACKAGE_NAME " is in initial sync and waiting for blocks...");
         }
     }