Update readme.txt

Author: vytisbulkevicius

readme.txt

@@ -2,7 +2,7 @@
 Contributors: optimole
 Tags: image optimization, convert webp, image optimizer, lazy load, image, optimisation, convert AVIF, resize, free cdn, compress images, optimize images
 Requires at least: 5.5
-Tested up to: 6.7
+Tested up to: 6.8
 Requires PHP: 7.4
 Stable tag: 4.0.0
 License: GPLv3