docs: document deprecated RPC and config flag removals in 0.21 release notes

Add entries to the Breaking Changes section of the 0.21 release notes
covering the RPCs, fields, and config flags removed in this PR that were
announced for removal in 0.21 via the 0.20 release notes.

Author: erickcestari

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

@@ -252,6 +252,46 @@
   `lncli getdebuginfo` and `lncli encryptdebugpackage` commands similarly
   require the `--include_log` flag to include logs in the output.
 
+* [Removed the deprecated payment and tracking
+  RPCs](https://github.com/lightningnetwork/lnd/pull/10795) that were
+  [announced for removal in 0.21](https://github.com/lightningnetwork/lnd/blob/master/docs/release-notes/release-notes-0.20.0.md#deprecations)
+  via the 0.20 release notes. Callers must migrate to the V2 equivalents:
+
+  | Removed RPC | Replacement |
+  |-------------|-------------|
+  | `lnrpc.SendPayment` (streaming) | `routerrpc.SendPaymentV2` |
+  | `lnrpc.SendPaymentSync` | `routerrpc.SendPaymentV2` |
+  | `lnrpc.SendToRoute` (streaming) | `routerrpc.SendToRouteV2` |
+  | `lnrpc.SendToRouteSync` | `routerrpc.SendToRouteV2` |
+  | `routerrpc.SendPayment` (streaming) | `routerrpc.SendPaymentV2` |
+  | `routerrpc.SendToRoute` | `routerrpc.SendToRouteV2` |
+  | `routerrpc.TrackPayment` (streaming) | `routerrpc.TrackPaymentV2` |
+
+  This also removes the corresponding REST routes
+  `POST /v1/channels/transaction-stream`, `POST /v1/channels/transactions`,
+  and `POST /v1/channels/transactions/route`. The orphan
+  `routerrpc.SendToRouteResponse` message (only used by the removed
+  `routerrpc.SendToRoute` RPC) is also dropped.
+
+* [Removed the deprecated `outgoing_chan_id`
+  field](https://github.com/lightningnetwork/lnd/pull/10795) from
+  `lnrpc.QueryRoutesRequest` and `routerrpc.SendPaymentRequest`. Proto tags
+  14 and 8 respectively are now reserved. Callers must use the multi-channel
+  `outgoing_chan_ids` field introduced in 0.20.
+
+* [Removed the deprecated `--tor.v2` configuration
+  flag](https://github.com/lightningnetwork/lnd/pull/10795). Tor v2 onion
+  services have been obsolete since October 2021 when the Tor network dropped
+  support for them; only v3 is supported now. Incoming
+  `NodeAnnouncement` messages still decode successfully, but any v2 onion
+  addresses they carry are stripped post-decode so they are never relayed
+  onward or persisted to the graph DB. The wire/storage encoders for v2
+  onion addresses (in
+  `lnwire`, `graph/db`) and the `tor.OnionHostToFakeIP` helper have been
+  removed. The `lnwire` and `graph/db` v2 decoders themselves are
+  retained so legacy graph databases (and the existing KV-to-SQL graph
+  migration that re-parses opaque address payloads) continue to load.
+
 ## Performance Improvements
 
 * Let the [channel graph cache be populated
@@ -331,7 +371,7 @@
 
 ### ⚠️ **Warning:** Deprecated fields in `lnrpc.Hop` will be removed in release version **0.22**
 
-  The following deprecated fields in the [`lnrpc.Hop`](https://lightning.engineering/api-docs/api/lnd/lightning/send-to-route-sync/#lnrpchop)
+  The following deprecated fields in the [`lnrpc.Hop`](https://lightning.engineering/api-docs/api/lnd/lightning/query-routes/#lnrpchop)
   message will be removed:
 
   | Field | Deprecated Since | Replacement |