### 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
@Override
        public boolean supportsDiffableValues() {
            return false;
        }
        @Override
        public Diff<V> diff(V value, V beforePart) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void writeDiff(Diff<V> value, StreamOutput out) {
            throw new UnsupportedOperationException();
        }
        @Override
        public Diff<V> readDiff(StreamInput in, K key) {
            throw new UnsupportedOperationException();
        }
    }
    /**
     * Implementation of the ValueSerializer that wraps value and diff readers.
     *
     * Note: this implementation is ignoring the key.
     */
    public static class DiffableValueReader<K, V extends Diffable<V>> extends DiffableValueSerializer<K, V> {