### 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 (routing != null) {
                if (routing.equals(aliasMd.indexRouting()) == false) {
                    throw new IllegalArgumentException(
                        "Alias ["
                            + aliasOrIndex
                            + "] has index routing associated with it ["
                            + aliasMd.indexRouting()
                            + "], and was provided with routing value ["
                            + routing
                            + "], rejecting operation"
                    );
                }
            }
            
            return aliasMd.indexRouting();
        }
        return routing;
    }
    private static void rejectSingleIndexOperation(String aliasOrIndex, IndexAbstraction result) {
        String[] indexNames = new String[result.getIndices().size()];
        int i = 0;
        for (Index indexName : result.getIndices()) {
            indexNames[i++] = indexName.getName();
        }
        throw new IllegalArgumentException(
            "Alias ["
                + aliasOrIndex
                + "] has more than one index associated with it ["