Rename confusing variable holding name of Logstash index template (#36840)

The previous name was confusing, so this commit clarifies it.

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

@@ -29,7 +29,7 @@ import java.util.regex.Pattern;
 public class Logstash extends Plugin implements ActionPlugin {
 
     private static final String LOGSTASH_TEMPLATE_FILE_NAME = "logstash-management";
-    private static final String LOGSTASH_INDEX_NAME = ".logstash-management";
+    private static final String LOGSTASH_INDEX_TEMPLATE_NAME = ".logstash-management";
     private static final String OLD_LOGSTASH_INDEX_NAME = "logstash-index-template";
     private static final String TEMPLATE_VERSION_PATTERN =
             Pattern.quote("${logstash.template.version}");
@@ -61,7 +61,7 @@ public class Logstash extends Plugin implements ActionPlugin {
     public UnaryOperator<Map<String, IndexTemplateMetaData>> getIndexTemplateMetaDataUpgrader() {
         return templates -> {
             templates.keySet().removeIf(OLD_LOGSTASH_INDEX_NAME::equals);
-            TemplateUtils.loadTemplateIntoMap("/" + LOGSTASH_TEMPLATE_FILE_NAME + ".json", templates, LOGSTASH_INDEX_NAME,
+            TemplateUtils.loadTemplateIntoMap("/" + LOGSTASH_TEMPLATE_FILE_NAME + ".json", templates, LOGSTASH_INDEX_TEMPLATE_NAME,
                     Version.CURRENT.toString(), TEMPLATE_VERSION_PATTERN, LogManager.getLogger(Logstash.class));
             return templates;
         };