Update Stable tag header

Author: aristath

readme.txt

@@ -4,7 +4,7 @@ Tags: planning, maintenance, writing, blogging
 Requires at least: 6.3
 Tested up to: 6.8
 Requires PHP: 7.4
-Stable tag: 1.8.1
+Stable tag: 1.9.0
 License: GPL3+
 License URI: https://www.gnu.org/licenses/gpl-3.0.en.html