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

@@ -10,6 +10,7 @@
     - [lncli Updates](#lncli-updates)
     - [Breaking Changes](#breaking-changes)
     - [Performance Improvements](#performance-improvements)
+    - [Deprecations](#deprecations)
 - [Technical and Architectural Updates](#technical-and-architectural-updates)
     - [BOLT Spec Updates](#bolt-spec-updates)
     - [Testing](#testing)
@@ -228,6 +229,26 @@ The underlying functionality between those two options remain the same.
 
 * Log rotation can now use ZSTD
 
+## Deprecations
+
+### ⚠️ **Warning:** The following RPCs will be removed in release version **0.21**:
+
+| Deprecated RPC Method | REST Equivalent | HTTP Method | Path | Replaced By |
+|----------------------|----------------|-------------|------------------------------|------------------|
+| [`lnrpc.SendToRoute`](https://lightning.engineering/api-docs/api/lnd/lightning/send-to-route/index.html) <br> [`routerrpc.SendToRoute`](https://lightning.engineering/api-docs/api/lnd/router/send-to-route/) | ❌ (No direct REST equivalent) | — | — | [`routerrpc.SendToRouteV2`](https://lightning.engineering/api-docs/api/lnd/router/send-to-route-v2/) |
+| [`lnrpc.SendPayment`](https://lightning.engineering/api-docs/api/lnd/lightning/send-payment/) <br> [`routerrpc.SendPayment`](https://lightning.engineering/api-docs/api/lnd/router/send-payment/) | ✅ | `POST` | `/v1/channels/transaction-stream` | [`routerrpc.SendPaymentV2`](https://lightning.engineering/api-docs/api/lnd/router/send-payment-v2/index.html) |
+| [`lnrpc.SendToRouteSync`](https://lightning.engineering/api-docs/api/lnd/lightning/send-to-route-sync/index.html) | ✅ | `POST` | `/v1/channels/transactions/route` | [`routerrpc.SendToRouteV2`](https://lightning.engineering/api-docs/api/lnd/router/send-to-route-v2/) |
+| [`lnrpc.SendPaymentSync`](https://lightning.engineering/api-docs/api/lnd/lightning/send-payment-sync/index.html) | ✅ | `POST` | `/v1/channels/transactions` | [`routerrpc.SendPaymentV2`](https://lightning.engineering/api-docs/api/lnd/router/send-payment-v2/index.html) |
+| [`router.TrackPayment`](https://lightning.engineering/api-docs/api/lnd/router/track-payment/index.html) | ❌ (No direct REST equivalent) | — | — | [`routerrpc.TrackPaymentV2`](https://lightning.engineering/api-docs/api/lnd/router/track-payment-v2/) |
+
+🚨 **Users are strongly encouraged** to transition to the new **V2 methods** before release **0.21** to ensure compatibility:
+
+| New RPC Method | REST Equivalent | HTTP Method | Path |
+|---------------|----------------|-------------|------------------------|
+| [`routerrpc.SendToRouteV2`](https://lightning.engineering/api-docs/api/lnd/router/send-to-route-v2/) | ✅ | `POST` | `/v2/router/route/send` |
+| [`routerrpc.SendPaymentV2`](https://lightning.engineering/api-docs/api/lnd/router/send-payment-v2/index.html) | ✅ | `POST` | `/v2/router/send` |
+| [`routerrpc.TrackPaymentV2`](https://lightning.engineering/api-docs/api/lnd/router/track-payment-v2/) | ✅ | `GET` | `/v2/router/track/{payment_hash}` |
+
 # Technical and Architectural Updates
 ## BOLT Spec Updates