Add 8.18.6 connectors release notes (#133635)

Author: charlotte-hoblik

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

@@ -42,6 +42,12 @@ PR https://github.com/elastic/connectors/pull/3577[*#3577*] resolves this issue
 
 There are no new features, enhancements, fixes, known issues, or deprecations associated with this release.
 
+[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