### 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
return (DataStreamMetadata) this.customs.getOrDefault(DataStreamMetadata.TYPE, DataStreamMetadata.EMPTY);
        }
        public boolean put(String aliasName, String dataStream, Boolean isWriteDataStream, String filter) {
            previousIndicesLookup = null;
            final DataStreamMetadata existing = dataStreamMetadata();
            final DataStreamMetadata updated = existing.withAlias(aliasName, dataStream, isWriteDataStream, filter);
            if (existing == updated) {
                return false;
            }
            this.customs.put(DataStreamMetadata.TYPE, updated);
            return true;
        }
        public Builder removeDataStream(String name) {
            previousIndicesLookup = null;
            this.customs.put(DataStreamMetadata.TYPE, dataStreamMetadata().withRemovedDataStream(name));
            return this;
        }
        public boolean removeDataStreamAlias(String aliasName, String dataStreamName, boolean mustExist) {
            previousIndicesLookup = null;
            final DataStreamMetadata existing = dataStreamMetadata();
            final DataStreamMetadata updated = existing.withRemovedAlias(aliasName, dataStreamName, mustExist);
            if (existing == updated) {
                return false;
            }
            this.customs.put(DataStreamMetadata.TYPE, updated);
            return true;
        }
        public Custom getCustom(String type) {
            return customs.get(type);
        }