[link-metrics] support at least one series per child (#7416)

Thread Conformance document requires a Link Metrics Subject to
support at least one Link Metrics series (forward or enhanced ACK)
per connected SED.

Make sure this is the default configuration.

examples/platforms/utils/link_metrics.cpp

@@ -153,7 +153,7 @@ private:
 
 enum
 {
-    kMaxEnhAckProbingInitiator = 10,
+    kMaxEnhAckProbingInitiator = OPENTHREAD_CONFIG_MLE_LINK_METRICS_MAX_SERIES_SUPPORTED,
 };
 
 typedef Pool<LinkMetricsDataInfo, kMaxEnhAckProbingInitiator> LinkMetricsDataInfoPool;