[Release] update version (#6005)

Co-authored-by: elasticmachine <[email protected]>

Author: elastic-vault-github-plugin-prod[bot]

version/version.go

@@ -6,4 +6,4 @@ package version
 
 // DefaultVersion is the current release version of Fleet-server, this version must match the
 // Elastic Agent version.
-const DefaultVersion = "9.1.8"
+const DefaultVersion = "9.1.9"