Update 'Tested up to' version to 6.9 in docs

Bumped the 'Tested up to' field from 6.6 to 6.9 in both readme.md and readme.txt to reflect compatibility with the latest WordPress version.

Author: pfefferle

readme.md

@@ -4,7 +4,7 @@
 **Tags:** indieweb, webmention, POSSE, indieauth  
 **Requires at least:** 4.7  
 **Requires PHP:** 5.6  
-**Tested up to:** 6.6  
+**Tested up to:** 6.9  
 **Stable tag:** 4.0.5  
 **License:** MIT  
 **License URI:** http://opensource.org/licenses/MIT  

readme.txt

@@ -4,7 +4,7 @@ Donate link: https://indieweb.org/how-to-sponsor
 Tags: indieweb, webmention, POSSE, indieauth
 Requires at least: 4.7
 Requires PHP: 5.6
-Tested up to: 6.6
+Tested up to: 6.9
 Stable tag: 4.0.5
 License: MIT
 License URI: http://opensource.org/licenses/MIT