Use consistent map type in IndicesService (#88369)

Iteration and lookup from this map get quite hot at times for large data nodes.
We shouldn't have different map types here depending on whether or not the latest operation
was an index delete or create on the node.
Also, the wrapped HashMap is slower to iterate than the immutable collections map.

server/src/main/java/org/elasticsearch/indices/IndicesService.java

@@ -164,8 +164,6 @@
 import java.util.stream.Collectors;
 
 import static java.util.Collections.emptyList;
-import static java.util.Collections.emptyMap;
-import static java.util.Collections.unmodifiableMap;
 import static org.elasticsearch.common.util.CollectionUtils.arrayAsArrayList;
 import static org.elasticsearch.common.util.concurrent.EsExecutors.daemonThreadFactory;
 import static org.elasticsearch.core.Strings.format;
@@ -218,7 +216,7 @@ public class IndicesService extends AbstractLifecycleComponent
     private final ScriptService scriptService;
     private final ClusterService clusterService;
     private final Client client;
-    private volatile Map<String, IndexService> indices = emptyMap();
+    private volatile Map<String, IndexService> indices = Map.of();
     private final Map<Index, List<PendingDelete>> pendingDeletes = new HashMap<>();
     private final AtomicInteger numUncompletedDeletes = new AtomicInteger();
     private final OldShardsStats oldShardsStats = new OldShardsStats();
@@ -860,10 +858,9 @@ public void removeIndex(final Index index, final IndexRemovalReason reason, fina
                 }
 
                 logger.debug("[{}] closing ... (reason [{}])", indexName, reason);
-                Map<String, IndexService> newIndices = new HashMap<>(indices);
-                indexService = newIndices.remove(index.getUUID());
+                indexService = indices.get(index.getUUID());
                 assert indexService != null : "IndexService is null for index: " + index;
-                indices = unmodifiableMap(newIndices);
+                indices = Maps.copyMapWithRemovedEntry(indices, index.getUUID());
                 listener = indexService.getIndexEventListener();
             }