fix: rename timestamp

proto/message_contents/frames.proto

@@ -18,7 +18,7 @@ message FrameActionBody {
   // The 1-indexed button that was clicked
   string button_index = 2;
   // Timestamp of the click in milliseconds since the epoch
-  uint64 timestamp_ms = 3;
+  uint64 timestamp = 3;
   // A unique identifier for the conversation, not tied to anything on the
   // network. Will not match the topic or conversation_id
   string opaque_conversation_identifier = 4;