fix: upgrade ember and drop node < v20 (#825)

This commit should trigger a major release. The actual upgrades
happended in 5515bc7

BREAKING CHANGE: drop support for node < 20 and ember < v4.12

Author: derrabauke

README.md

@@ -9,8 +9,7 @@ The frontend for the [emeis](https://github.com/projectcaluma/emeis) user manage
 
 ## Compatibility
 
-- Ember.js v5.8 or above
-- Ember CLI v5.8 or above
+- Ember.js v4.12 or above
 - Node.js v20 or above
 
 ## Installation