documentation: Incorporate CHANGELOG.md notes from v6 1.21.1 branch

Author: rbasamoyai

CHANGELOG.md

@@ -8,6 +8,9 @@ Changes:
 - `block_hit_effects` files have changed format
     - now only accept `impact_particles` and `deflect_particles` lists
     - particles formatted in JSON rather than command string
+- [Developers, NeoForge] `CBCNeoForgeRegisterEvent` has been removed in favor of accessing CBC registries via
+  `RegisterEvent`.
+- Very old CBC objects will no longer be supported.
 Fixes:
 - Fixed gold sheet tag and dependent recipes
 - Fixed wrong id on nitropowder milling recipe