Update tested up to version in README

Author: pfefferle

README.md

@@ -4,7 +4,7 @@
 - Donate link: https://notiz.blog/donate/
 - Tags: nodeinfo, fediverse, ostatus, diaspora, activitypub
 - Requires at least: 4.9
-- Tested up to: 6.7
+- Tested up to: 6.9
 - Stable tag: 2.3.1
 - Requires PHP: 5.6
 - License: MIT