Deprecate history field

Author: alexshtin

temporal/api/history/v1/message.proto

@@ -472,7 +472,8 @@ message WorkflowExecutionSignaledEventAttributes {
     // Headers that were passed by the sender of the signal and copied by temporal 
     // server into the workflow task.
     temporal.api.common.v1.Header header = 4;
-    reserved 5;
+    // This field is deprecated and never respected. It should always be set to false.
+    bool skip_generate_workflow_task = 5 [deprecated = true];
     // When signal origin is a workflow execution, this field is set.
     temporal.api.common.v1.WorkflowExecution external_workflow_execution = 6;
 }