### 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
if (indexAliases.contains(dataStreamAlias)) {
                    duplicates.add("data stream alias and indices alias have the same name (" + dataStreamAlias + ")");
                }
                if (indicesMap.containsKey(dataStreamAlias)) {
                    aliasDuplicatesWithIndices.add(dataStreamAlias);
                }
                if (allDataStreams.containsKey(dataStreamAlias)) {
                    aliasDuplicatesWithDataStreams.add(dataStreamAlias);
                }
            }
            for (String alias : indexAliases) {
                if (allDataStreams.containsKey(alias)) {
                    aliasDuplicatesWithDataStreams.add(alias);
                }
                if (indicesMap.containsKey(alias)) {
                    aliasDuplicatesWithIndices.add(alias);
                }
            }
            allDataStreams.forEach((key, value) -> {
                if (indicesMap.containsKey(key)) {
                    duplicates.add("data stream [" + key + "] conflicts with index");
                }
            });
            if (aliasDuplicatesWithIndices.isEmpty() == false) {
                collectAliasDuplicates(indicesMap, aliasDuplicatesWithIndices, duplicates);
            }
            if (aliasDuplicatesWithDataStreams.isEmpty() == false) {
                collectAliasDuplicates(indicesMap, dataStreamMetadata, aliasDuplicatesWithDataStreams, duplicates);
            }
            if (duplicates.isEmpty() == false) {