Fix MultiClustersIT#testStats (elastic#119494)

This test depends on `/_capabilities` endpoint introduced in 8.15.
Sometimes multi-cluster is executed against older versions on CI.
This change ensures the test is executed only on versions with the endpoint.

x-pack/plugin/esql/qa/server/multi-clusters/src/javaRestTest/java/org/elasticsearch/xpack/esql/ccq/MultiClustersIT.java

@@ -400,6 +400,8 @@ public void testIndexPattern() throws Exception {
 
     @SuppressWarnings("unchecked")
     public void testStats() throws IOException {
+        assumeTrue("capabilities endpoint is not available", capabilitiesEndpointAvailable());
+
         Request caps = new Request("GET", "_capabilities?method=GET&path=_cluster/stats&capabilities=esql-stats");
         Response capsResponse = client().performRequest(caps);
         Map<String, Object> capsResult = entityAsMap(capsResponse.getEntity());
@@ -439,6 +441,10 @@ private static boolean ccsMetadataAvailable() {
         return Clusters.localClusterVersion().onOrAfter(Version.V_8_16_0);
     }
 
+    private static boolean capabilitiesEndpointAvailable() {
+        return Clusters.localClusterVersion().onOrAfter(Version.V_8_15_0);
+    }
+
     private static boolean includeCCSMetadata() {
         return ccsMetadataAvailable() && randomBoolean();
     }