fix!: Replace unreleased InitialSubscribeRequest initial_cursor field…

… with initial_location

PiperOrigin-RevId: 375562697

google/cloud/pubsublite/v1/subscriber.proto

@@ -67,11 +67,10 @@ message InitialSubscribeRequest {
   // so `partition` must be in the range [0, topic.num_partitions).
   int64 partition = 2;
 
-  // Optional. Initial stream delivery cursor, pointing to anywhere in the topic
-  // partition. Cursors past head result in stream breakage. If not set,
+  // Optional. Initial target location within the message backlog. If not set,
   // messages will be delivered from the commit cursor for the given
   // subscription and partition.
-  Cursor initial_cursor = 3 [(google.api.field_behavior) = OPTIONAL];
+  SeekRequest initial_location = 4 [(google.api.field_behavior) = OPTIONAL];
 }
 
 // Response to an InitialSubscribeRequest.
@@ -82,9 +81,11 @@ message InitialSubscribeResponse {
 }
 
 // Request to update the stream's delivery cursor based on the given target.
-// Resets the server available tokens to 0. SeekRequests may not be sent while
-// another SeekRequest is outstanding (i.e., has not received a SeekResponse) on
-// the same stream. SeekRequests past head result in stream breakage.
+// Resets the server available tokens to 0. SeekRequests past head result in
+// stream breakage.
+//
+// SeekRequests may not be sent while another SeekRequest is outstanding (i.e.,
+// has not received a SeekResponse) on the same stream.
 message SeekRequest {
   // A special target in the partition that takes no other parameters.
   enum NamedTarget {