docs: update release-notes-0.19.0.md

In this commit, we warn users about the removal
of RPCs `SendToRoute`, `SendToRouteSync`, `SendPayment`,
and `SendPaymentSync` in the next release 0.20.

Author: mohamedawnallah

docs/release-notes/release-notes-0.19.0.md

@@ -224,6 +224,16 @@ The underlying functionality between those two options remain the same.
 
 
 ## Breaking Changes
+
+* **Warning:** The following RPCs will be removed in the next release (0.20):
+  * `SendToRoute` → Use `SendToRouteV2` instead.
+  * `SendToRouteSync` → Use `SendToRouteV2` instead.
+  * `SendPayment` → Use `SendPaymentV2` instead.
+  * `SendPaymentSync` → Use `SendPaymentV2` instead.
+
+  Users are strongly encouraged to transition to the new V2 methods before the
+  next release to ensure compatibility.
+
 ## Performance Improvements
 
 * Log rotation can now use ZSTD