Apply suggestion from @aristath

Author: aristath

readme.txt

@@ -1,7 +1,7 @@
 === Progress Planner ===
 Contributors: joostdevalk, aristath, filipi, jonoaldersonwp, mariekerakt, irisguelen, samalderson, tacoverdo
 Tags: planning, maintenance, writing, blogging
-Requires at least: 6.6
+Requires at least: 6.7
 Tested up to: 6.9
 Requires PHP: 7.4
 Stable tag: 1.9.0