docs: update release notes

Update the Database section to reference both PRs that prepare the
graph DB for gossip v2 support:
- PR 10339: node handling
- PR 10379: channel handling (this PR)

Author: ellemouton

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

@@ -68,8 +68,9 @@
 * Freeze the [graph SQL migration 
   code](https://github.com/lightningnetwork/lnd/pull/10338) to prevent the 
   need for maintenance as the sqlc code evolves. 
-* Prepare the graph DB for handling gossip [V2 
-  nodes](https://github.com/lightningnetwork/lnd/pull/10339). 
+* Prepare the graph DB for handling gossip V2 
+  nodes and channels [1](https://github.com/lightningnetwork/lnd/pull/10339)
+    [2](https://github.com/lightningnetwork/lnd/pull/10379).
 
 ## Code Health