Adapt GHA badge after cutting the release branch

Author: abias

README.md

@@ -1,7 +1,7 @@
 moodle-atto_styles
 ==================
 
-[![Moodle Plugin CI](https://github.com/moodle-an-hochschulen/moodle-atto_styles/workflows/Moodle%20Plugin%20CI/badge.svg?branch=main)](https://github.com/moodle-an-hochschulen/moodle-atto_styles/actions?query=workflow%3A%22Moodle+Plugin+CI%22+branch%3Amain)
+[![Moodle Plugin CI](https://github.com/moodle-an-hochschulen/moodle-atto_styles/workflows/Moodle%20Plugin%20CI/badge.svg?branch=MOODLE_404_STABLE)](https://github.com/moodle-an-hochschulen/moodle-atto_styles/actions?query=workflow%3A%22Moodle+Plugin+CI%22+branch%3AMOODLE_404_STABLE)
 
 Moodle Atto plugin which lets the administrator provide custom CSS classes which can afterwards be applied by users to block and inline text in Atto