Fix docs URLs to use '8.0' instead of 'master'

Author: sethmlarson

docs/observability.asciidoc

@@ -377,9 +377,9 @@ child.search({
 To improve observability, the client offers an easy way to configure the 
 `X-Opaque-Id` header. If you set the `X-Opaque-Id` in a specific request, this 
 allows you to discover this identifier in the 
-https://www.elastic.co/guide/en/elasticsearch/reference/master/logging.html#deprecation-logging[deprecation logs], 
-helps you with https://www.elastic.co/guide/en/elasticsearch/reference/master/index-modules-slowlog.html#_identifying_search_slow_log_origin[identifying search slow log origin] 
-as well as https://www.elastic.co/guide/en/elasticsearch/reference/master/tasks.html#_identifying_running_tasks[identifying running tasks].
+https://www.elastic.co/guide/en/elasticsearch/reference/8.0/logging.html#deprecation-logging[deprecation logs], 
+helps you with https://www.elastic.co/guide/en/elasticsearch/reference/8.0/index-modules-slowlog.html#_identifying_search_slow_log_origin[identifying search slow log origin] 
+as well as https://www.elastic.co/guide/en/elasticsearch/reference/8.0/tasks.html#_identifying_running_tasks[identifying running tasks].
 
 The `X-Opaque-Id` should be configured in each request, for doing that you can 
 use the `opaqueId` option, as you can see in the following example. The