### Description

When using the DiffableUtils.diff with a vanilla OpenJDK Map, instead of the legacy ImmutableOpenMap interface, we return a mutable map (generic HashMap), which isn't the expected behaviour of the API:

https://github.com/elastic/elasticsearch/blob/45c56b22f946733712ad6d92600411c0885594c5/server/src/main/java/org/elasticsearch/cluster/DiffableUtils.java#L85

We should wrap the map returned by the diff API in an unmodifiable version.
server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch.cluster.metadata;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.lucene.util.CollectionUtil;
import org.elasticsearch.Version;
import org.elasticsearch.cluster.ClusterState;
import org.elasticsearch.cluster.Diff;
import org.elasticsearch.cluster.Diffable;
import org.elasticsearch.cluster.DiffableUtils;
import org.elasticsearch.cluster.NamedDiffable;
import org.elasticsearch.cluster.NamedDiffableValueSerializer;
import org.elasticsearch.cluster.block.ClusterBlock;
import org.elasticsearch.cluster.block.ClusterBlockLevel;
import org.elasticsearch.cluster.coordination.CoordinationMetadata;
import org.elasticsearch.cluster.metadata.IndexAbstraction.ConcreteIndex;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.UUIDs;
import org.elasticsearch.common.collect.ImmutableOpenMap;
import org.elasticsearch.common.io.stream.StreamInput;
import org.elasticsearch.common.io.stream.StreamOutput;