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 +252,33 @@
   `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 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`.
+
+* Removed the deprecated `outgoing_chan_id` field from `QueryRoutesRequest`
+  and `SendPaymentRequest`. Callers must use the multi-channel
+  `outgoing_chan_ids` field introduced in 0.20. The compatibility fallback
+  that accepted either field (and errored when both were set) is gone.
+
+* Removed the deprecated `--tor.v2` configuration flag. Tor v2 onion services
+  have been obsolete since October 2021 when the Tor network dropped support
+  for them; only v3 is supported now.
+
 ## Performance Improvements
 
 * Let the [channel graph cache be populated