Mark Elasticsearch 6 support for synonyms

Author: aapokiiso

app/code/Magento/Elasticsearch6/etc/search_engine.xml

@@ -0,0 +1,12 @@
+<?xml version="1.0"?>
+<!--
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+-->
+<engines xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Search/etc/search_engine.xsd">
+    <engine name="elasticsearch6">
+        <feature name="synonyms" support="true" />
+    </engine>
+</engines>