feat: remove target originator (#277)

Author: mkysel

proto/xmtpv4/envelopes/envelopes.proto

@@ -17,7 +17,7 @@ message Cursor {
 
 // Data visible to the server that has been authenticated by the client.
 message AuthenticatedData {
-  optional uint32 target_originator = 1 [deprecated = true];
+  // Do NOT reuse tag 1 — previously used by target_originator
   bytes target_topic = 2;
   Cursor depends_on = 3;
   bool is_commit = 4;