lnrpc: deprecate unused pending closing channels field

The pending closing channels field was present to cover an edge case
where coperatively closed channels were closed before upgrade and
confirmed after the upgrade. This commit deprecates the field and adds
a warning log to cover these edge cases.

lnrpc/rpc.proto

@@ -2096,8 +2096,12 @@ message PendingChannelsResponse {
     /// Channels pending opening
     repeated PendingOpenChannel pending_open_channels = 2;
 
-    /// Channels pending closing
-    repeated ClosedChannel pending_closing_channels = 3;
+    /*
+    Deprecated: Channels pending closing previously contained cooperatively
+    closed channels with a single confirmation. These channels are now
+    considered closed from the time we see them on chain.
+    */
+    repeated ClosedChannel pending_closing_channels = 3 [deprecated = true];
 
     /// Channels pending force closing
     repeated ForceClosedChannel pending_force_closing_channels = 4;

lnrpc/rpc.swagger.json

@@ -3939,7 +3939,7 @@
           "items": {
             "$ref": "#/definitions/PendingChannelsResponseClosedChannel"
           },
-          "title": "/ Channels pending closing"
+          "description": "Deprecated: Channels pending closing previously contained cooperatively\nclosed channels with a single confirmation. These channels are now\nconsidered closed from the time we see them on chain."
         },
         "pending_force_closing_channels": {
           "type": "array",