fix(rlpx): gate IPv6 dual-stack TCP binding on DiscV5

DiscV4 cannot advertise IPv6 addresses so binding a second RLPx TCP
socket with DiscV4 serves no useful purpose and could surprise operators
expecting unchanged behaviour.

Only enable the IPv6 RLPx socket when both discovery and DiscV5 are
active. The guard can be removed once DiscV4 is deprecated and removed.

Signed-off-by: Usman Saleem <usman@usmans.info>

Author: usmansaleem

app/src/main/java/org/hyperledger/besu/RunnerBuilder.java

@@ -706,12 +706,20 @@ public Runner build() {
             .flatMap(protocolManager -> protocolManager.getSupportedCapabilities().stream())
             .collect(Collectors.toSet());
 
+    // Only enable RLPx IPv6 dual-stack when DiscV5 is active. DiscV4 cannot advertise IPv6
+    // addresses, so binding a second TCP socket would serve no purpose and could surprise
+    // operators. When DiscV4 is eventually removed this guard can be dropped.
+    final boolean rlpxDualStackEnabled =
+        discoveryEnabled && networkingConfiguration.discoveryConfiguration().isDiscoveryV5Enabled();
     final RlpxConfiguration rlpxConfiguration =
         RlpxConfiguration.create()
             .setBindHost(p2pListenInterface)
             .setBindPort(p2pListenPort)
-            .setBindHostIpv6(p2pListenInterfaceIpv6)
-            .setBindPortIpv6(p2pListenInterfaceIpv6.map(ignored -> p2pListenPortIpv6))
+            .setBindHostIpv6(rlpxDualStackEnabled ? p2pListenInterfaceIpv6 : Optional.empty())
+            .setBindPortIpv6(
+                rlpxDualStackEnabled
+                    ? p2pListenInterfaceIpv6.map(ignored -> p2pListenPortIpv6)
+                    : Optional.empty())
             .setSupportedProtocols(subProtocols)
             .setClientId(BesuVersionUtils.nodeName(identityString));
     networkingConfiguration =