docs: add release-notes

Author: ziggie1984

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

@@ -87,6 +87,8 @@
     db functions Part 1](https://github.com/lightningnetwork/lnd/pull/10307)
   * [Thread context through payment 
     db functions Part 2](https://github.com/lightningnetwork/lnd/pull/10308)
+  * [Finalize SQL implementation for 
+    payments db](https://github.com/lightningnetwork/lnd/pull/10373)
 
 
 ## Code Health