Update logstash-management.json to use typeless template (elastic#38653)

This commit changes the type from "doc" to "_doc" for the
.logstash-management template. Since this is an internally
managed template it does not always go through the REST
layer for it's internal representation.  The internal
representation requires the default "_doc" type, which for
external templates is added in the REST layer.

Related elastic#38637

x-pack/plugin/core/src/main/resources/logstash-management.json

@@ -8,7 +8,7 @@
     }
   },
   "mappings" : {
-    "doc" : {
+    "_doc" : {
       "_meta": {
         "logstash-version": "${logstash.template.version}"
       },

x-pack/plugin/logstash/src/main/java/org/elasticsearch/xpack/logstash/Logstash.java

@@ -10,6 +10,7 @@
 import org.elasticsearch.cluster.metadata.IndexTemplateMetaData;
 import org.elasticsearch.common.inject.Module;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.plugins.ActionPlugin;
 import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.xpack.core.XPackPlugin;
@@ -63,6 +64,8 @@ public UnaryOperator<Map<String, IndexTemplateMetaData>> getIndexTemplateMetaDat
             templates.keySet().removeIf(OLD_LOGSTASH_INDEX_NAME::equals);
             TemplateUtils.loadTemplateIntoMap("/" + LOGSTASH_TEMPLATE_FILE_NAME + ".json", templates, LOGSTASH_INDEX_TEMPLATE_NAME,
                     Version.CURRENT.toString(), TEMPLATE_VERSION_PATTERN, LogManager.getLogger(Logstash.class));
+            //internal representation of typeless templates requires the default "_doc" type, which is also required for internal templates
+            assert templates.get(LOGSTASH_INDEX_TEMPLATE_NAME).mappings().get(MapperService.SINGLE_MAPPING_NAME) != null;
             return templates;
         };
     }