Archive old changelog

This reverts commit 62d1f0acbd and moves
the current changelog to CHANGELOG-OLD.md.

The current format is pretty messy and wasn't really much better than a
git shortlog. Changelogs are meant for end-users to know what changed
between versions. Most probably won't care much about refactors but may
be interested in UI changes, feature additions/removals, and maybe
client API changes (for 3rd party clients).

CHANGELOG-OLD.md

@@ -15,8 +15,6 @@ You should also include the user name that made the change.
 - ハイライトがみつけるに統合されました
 - カスタム絵文字ページはインスタンス情報ページに統合されました
 - 連合ページはインスタンス情報ページに統合されました
-- Remove ads @Johann150
-- Simplify ID generation: Only the `aid` generation algorithm will be supported going forward. @Johann150
 
 ### Improvements
 - Client: Fix URL-encoded routing
@@ -34,10 +32,6 @@ You should also include the user name that made the change.
 - Server: Improve performance
 - Server: Supports IPv6 on Redis transport. @mei23  
   IPv4/IPv6 is used by default. You can tune this behavior via `redis.family`.
-- Backend: Improve mutes and blocks @toast
-- Hide metadata of private notes @Johann150
-- Add padding to pages @norm
-- Emoji picker is now case-insensitive @Johann150
 
 ### Bugfixes
 - Server: Fix GenerateVideoThumbnail failed @mei23