Increment version on system index settings change (elastic#84994)

The test was failing on assertion around index settings version change
in case actual settings were changed. This change adds missing version
increment and readable messages to the related assertions.

Author: idegtiarenko

server/src/main/java/org/elasticsearch/cluster/metadata/SystemIndexMetadataUpgradeService.java

@@ -104,6 +104,7 @@ public ClusterState execute(ClusterState currentState) throws Exception {
                     boolean isHidden = indexMetadata.getSettings().getAsBoolean(IndexMetadata.SETTING_INDEX_HIDDEN, false);
                     if (isSystem && isHidden == false) {
                         builder.settings(Settings.builder().put(indexMetadata.getSettings()).put(IndexMetadata.SETTING_INDEX_HIDDEN, true));
+                        builder.settingsVersion(builder.settingsVersion() + 1);
                         updated = true;
                     }
                     if (isSystem && indexMetadata.getAliases().values().stream().anyMatch(a -> Boolean.FALSE.equals(a.isHidden()))) {

server/src/main/java/org/elasticsearch/index/IndexService.java

@@ -784,9 +784,9 @@ public synchronized void updateMetadata(final IndexMetadata currentIndexMetadata
             final long currentSettingsVersion = currentIndexMetadata.getSettingsVersion();
             final long newSettingsVersion = newIndexMetadata.getSettingsVersion();
             if (currentSettingsVersion == newSettingsVersion) {
-                assert updateIndexSettings == false;
+                assert updateIndexSettings == false : "No index updates are expected as index settings version has not changed";
             } else {
-                assert updateIndexSettings;
+                assert updateIndexSettings : "Index updates are expected as index settings version has changed";
                 assert currentSettingsVersion < newSettingsVersion
                     : "expected current settings version ["
                         + currentSettingsVersion