Add v5.12.0 to CHANGELOG for ember-source

(cherry picked from commit eacea65)

Author: kategengler

CHANGELOG.md

@@ -1,15 +1,8 @@
 # Ember Changelog
 
-## v5.12.0-beta.3 (September 23, 2024)
+## v5.12.0 (September 30, 2024)
 
 - [#20746](https://github.com/emberjs/ember.js/pull/20746) [BUGFIX] Upgrade glimmer-vm for some perf improvements
-
-## v5.12.0-beta.2 (September 16, 2024)
-
-- [#20734](https://github.com/emberjs/ember.js/pull/20734) Circularity fixes
-
-## v5.12.0-beta.1 (August 19, 2024)
-
 - [#20714](https://github.com/emberjs/ember.js/pull/20714) [BUGFIX] Break eager cycle for "deprecate" function
 
 ## v5.11.1 (September 23, 2024)