docs: add release-notes

Author: ziggie1984

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

@@ -184,6 +184,10 @@
     db functions Part 2](https://github.com/lightningnetwork/lnd/pull/10308)
   * [Finalize SQL implementation for 
     payments db](https://github.com/lightningnetwork/lnd/pull/10373)
+  * [Add the KV-to-SQL payment
+    migration](https://github.com/lightningnetwork/lnd/pull/10485) with 
+    comprehensive tests and build tag "test_native_sql" gated wiring into the 
+    payment flow.
 
 
 ## Code Health