Add release notes for 8.9.1 (#7867)

Author: flobernd

docs/release-notes/release-notes-8.9.1.asciidoc

@@ -0,0 +1,7 @@
+[[release-notes-8.9.1]]
+== Release notes v8.9.1
+
+[discrete]
+=== Bug fixes
+
+- https://github.com/elastic/elasticsearch-net/pull/7864[#7864] Fix `TextExpansionQuery` definition

docs/release-notes/release-notes.asciidoc

@@ -6,6 +6,7 @@
 [discrete]
 == Version 8.9
 
+* <<release-notes-8.9.1,Release notes v8.9.1>>
 * <<release-notes-8.9.0,Release notes v8.9.0>>
 
 [discrete]
@@ -32,6 +33,7 @@
 * <<release-notes-8.0.0,Release notes v8.0.0>>
 
 include::breaking-change-policy.asciidoc[]
+include::release-notes-8.9.1.asciidoc[]
 include::release-notes-8.9.0.asciidoc[]
 include::release-notes-8.1.3.asciidoc[]
 include::release-notes-8.1.2.asciidoc[]