Add 8.18.6 connectors release notes

Author: charlotte-hoblik

docs/reference/connector/docs/connectors-release-notes.asciidoc

@@ -9,6 +9,12 @@
 Prior to version *8.16.0*, the connector release notes were published as part of the {enterprise-search-ref}/changelog.html[Enterprise Search documentation].
 ====
 
+[discrete]
+[[es-connectors-release-notes-8-18-6]]
+=== 8.18.6
+
+No notable changes in this release.
+
 [discrete]
 [[es-connectors-release-notes-8-18-5]]
 === 8.18.5