Add Requires PHP header and bump tested-up-to version

Author: pfefferle

readme.md

@@ -3,8 +3,9 @@
 - Contributors: indieweb, pfefferle, dshanske
 - Donate link: https://opencollective.com/indieweb
 - Tags: IndieAuth, IndieWeb, login, oauth
-- Tested up to: 6.9
+- Tested up to: 7.0
 - Stable tag: 4.5.5
+- Requires PHP: 7.4
 - License: MIT
 - License URI: https://opensource.org/licenses/MIT