docs: add release note for cross-version graph Store

Author: ellemouton

docs/release-notes/release-notes-0.21.0.md

@@ -316,6 +316,12 @@
   The v1 end-time bound is corrected from inclusive to exclusive to match the
   BOLT 07 `gossip_timestamp_filter` spec. New SQL queries and composite indexes
   are added for efficient v2 block-height range scans.
+* Make the [graph `Store` interface
+  cross-version](https://github.com/lightningnetwork/lnd/pull/10656) so that
+  `ForEachNode`, `ForEachChannel`, and `ForEachNodeDirectedChannel` work across
+  gossip v1 and v2. Add `PreferHighest` fetch helpers and `GetVersions` queries
+  so callers can retrieve channels without knowing which gossip version
+  announced them.
 * Updated waiting proof persistence for gossip upgrades by introducing typed
   waiting proof keys and payloads, with a DB migration to rewrite legacy
   waiting proof records to the new key/value format