message: Added since to fixed description
diff: * Fix a data race reported by thread sanitizer when preparing to deliver change notifications. This probably did not cause observable problems in practice ([PR #5892](https://github.com/realm/realm-core/pull/5892) since 12.7.0).
  * Changed the behaviour of creating a collection of non-nullable Mixed type to throw a descriptive error message rather than asserting in debug mode. ([#5894](https://github.com/realm/realm-core/issues/5894), since the introduction of the Mixed type).
  * Fix a use-after-free when a sync session is closed and the app is destroyed at the same time ([#5752](https://github.com/realm/realm-core/issues/5752), since v11.5.2).
  -* Fix Http transport doesn't correctly preserve the request body ([#5890](https://github.com/realm/realm-core/issues/5890)).
  +* Fix Http transport doesn't correctly preserve the request body ([#5890](https://github.com/realm/realm-core/issues/5890), since 12.8.0).
  ### Breaking changes
  * Removed breaking callback changes for `GenericNetworkTransport::send_request_to_server()` ([PR #5898](https://github.com/realm/realm-core/pull/5898)).
  