bitcoind: Migrate warning string to list of warnings

The `warning` string was replaced by a `warnings` JSON array in Bitcoin
Core 25.0.

wallets/bitcoind/src/main/java/bisq/wallets/bitcoind/rpc/responses/BitcoindWarningResponse.java

@@ -20,13 +20,16 @@
 import bisq.wallets.json_rpc.JsonRpcResponse;
 import lombok.Getter;
 
+import java.util.Collections;
+import java.util.List;
+
 public class BitcoindWarningResponse extends JsonRpcResponse<BitcoindWarningResponse.Result> {
     @Getter
     public static class Result {
-        protected String warning;
+        protected List<String> warnings = Collections.emptyList();
 
         public boolean hasWarning() {
-            return !warning.isEmpty();
+            return !warnings.isEmpty();
         }
     }
 }