docs: release note for chanstate Store interface

Seeds the v0.22 "Native SQL migration of the channel state and switch
data" series under the Database header so subsequent decomposition PRs
(consumer migration, KV/SQL stores, revocation log, forwarding package,
etc.) can hang off the same heading as nested bullets, mirroring the
v0.21 payments series shape.

Author: ziggie1984

docs/release-notes/release-notes-0.22.0.md

@@ -51,6 +51,12 @@
 
 ## Database
 
+* Native SQL migration of the channel state and switch data:
+  * Introduce the [`chanstate.Store`
+    interface](https://github.com/lightningnetwork/lnd/pull/10777) as the
+    persistence contract for the channel-state subsystem, decoupling
+    consumers from `*channeldb.ChannelStateDB` ahead of the SQL migration.
+
 ## Code Health
 
 ## Tooling and Documentation