Move release note to unreleased changes

Adding `PlayerTransferState` didn't go into 1.8.0-beta01 release, but the release note was kept there by mistake. It also wasn't cherry-picked into 1.8.0-rc01, so moving it to Unreleased notes section.

PiperOrigin-RevId: 780118060
(cherry picked from commit a80f551)

Author: shahdDaghash

RELEASENOTES.md

@@ -64,8 +64,6 @@ This release includes the following changes since the
 
 *   Common Library:
     *   Add support for replacing the player in `ForwardingSimpleBasePlayer`.
-    *   Add `PlayerTransferState`, which facilitates transferring the playback
-        state across `Player` instances.
 *   ExoPlayer:
     *   Add support for using the virtual device ID from the `Context` passed to
         `ExoPlayer.Builder`.