[doc] add version of C-core for xDS dualstack support (#37662)

Closes #37662

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37662 from markdroth:xds_features_dualstack f6b69e3048
PiperOrigin-RevId: 678277887

doc/grpc_xds_features.md

@@ -82,3 +82,4 @@ mTLS Credentials in xDS Bootstrap File | [A65](https://github.com/grpc/proposal/
 Stateful Session Affinity | [A55](https://github.com/grpc/proposal/blob/master/A55-xds-stateful-session-affinity.md), [A60](https://github.com/grpc/proposal/blob/master/A60-xds-stateful-session-affinity-weighted-clusters.md), [A75](https://github.com/grpc/proposal/blob/master/A75-xds-aggregate-cluster-behavior-fixes.md) | v1.61.0 | | | |
 xDS Locality label for OpenTelemetry metrics | [A78](https://github.com/grpc/proposal/blob/master/A78-grpc-metrics-wrr-pf-xds.md) | v1.63.0 (C++) | v1.64.0 | | |
 xDS Fallback | [A71](https://github.com/grpc/proposal/blob/master/A71-xds-fallback.md) | v1.67.0 | | | |
+Dualstack Backend Support | [A61](https://github.com/grpc/proposal/blob/master/A61-IPv4-IPv6-dualstack-backends.md) | v1.66.1 | | | |