### 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/DiffableUtils.java
private DiffableUtils() {}
    private static final MapDiff<?, ?, ?> EMPTY = new MapDiff<>(null, null, List.of(), List.of(), List.of()) {
        @Override
        public Object apply(Object part) {
            return part;
        }
    };
    /**
     * Returns a map key serializer for String keys
     */
    public static KeySerializer<String> getStringKeySerializer() {
        return StringKeySerializer.INSTANCE;
    }