docs: add release-notes

Author: ziggie1984

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

@@ -58,6 +58,8 @@
     SQL Backend](https://github.com/lightningnetwork/lnd/pull/10287)
   * Implement insert methods for the [payments db 
     SQL Backend](https://github.com/lightningnetwork/lnd/pull/10291)
+  * Finalize SQL payments implementation [enabling unit and itests
+    for SQL backend](https://github.com/lightningnetwork/lnd/pull/10292)
 
 ## Code Health