### 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
public DataStream dataStream(String dataStreamName) {
            return dataStreamMetadata().dataStreams().get(dataStreamName);
        }
        public Builder dataStreams(Map<String, DataStream> dataStreams, Map<String, DataStreamAlias> dataStreamAliases) {
            previousIndicesLookup = null;
            // Only perform data stream validation only when data streams are modified in Metadata:
            for (DataStream dataStream : dataStreams.values()) {
                dataStream.validate(indices::get);
            }
            this.customs.put(
                DataStreamMetadata.TYPE,
                new DataStreamMetadata(
                    ImmutableOpenMap.<String, DataStream>builder().putAllFromMap(dataStreams).build(),
                    ImmutableOpenMap.<String, DataStreamAlias>builder().putAllFromMap(dataStreamAliases).build()
                )
            );
            return this;
        }
        public Builder put(DataStream dataStream) {
            previousIndicesLookup = null;
            Objects.requireNonNull(dataStream, "it is invalid to add a null data stream");
            // Every time the backing indices of a data stream is modified a new instance will be created and
            // that instance needs to be added here. So this is a good place to do data stream validation for
            // the data stream and all of its backing indices. Doing this validation in the build() method would
            // trigger this validation on each new Metadata creation, even if there are no changes to data streams.
            dataStream.validate(indices::get);
            this.customs.put(DataStreamMetadata.TYPE, dataStreamMetadata().withAddedDatastream(dataStream));
            return this;
        }
        public DataStreamMetadata dataStreamMetadata() {