Bump "Tested up to" version to 6.9

Author: ilicfilip

readme.txt

@@ -2,7 +2,7 @@
 Contributors: joostdevalk
 Tags: tag, tags, seo
 Requires at least: 6.2
-Tested up to: 6.6
+Tested up to: 6.9
 Requires PHP: 7.4
 Stable tag: 1.5.1
 License: GPL3+