Update changelog

CHANGELOG.md

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+# v0.12.2 (unreleased)
+
+### Added
+* Added built-in custom emoji packs to allow reacting with any standard unicode
+  emoji from Matrix (note that only premium users can use custom emojis).
+* Added infinite backfill using [MSC2716].
+  * The new system includes a backwards compatibility mechanism which uses the
+    old method of just sending events to the room. By default, MSC2716 is not
+    enabled and the legacy method will be used.
+
+### Improved
+* Redacting reactions on Matrix no longer removes the user's other reactions to
+  the same message (premium users can have up to 3 reactions per message).
+
+### Fixed
+
+[MSC2716]: https://github.com/matrix-org/matrix-spec-proposals/pull/2716
+
 # v0.12.1 (2022-09-26)
 
 ### Added