### 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 static final StringKeySerializer INSTANCE = new StringKeySerializer();
        @Override
        public void writeKey(String key, StreamOutput out) throws IOException {
            out.writeString(key);
        }
        @Override
        public String readKey(StreamInput in) throws IOException {
            return in.readString();
        }
    }
    /**
     * Serializes Integer keys of a map as an Int
     */
    private static final class IntKeySerializer implements KeySerializer<Integer> {
        public static final IntKeySerializer INSTANCE = new IntKeySerializer();
        @Override
        public void writeKey(Integer key, StreamOutput out) throws IOException {
            out.writeInt(key);
        }
        @Override
        public Integer readKey(StreamInput in) throws IOException {
            return in.readInt();
        }
    }
    /**
     * Serializes Integer keys of a map as a VInt. Requires keys to be positive.
     */
    private static final class VIntKeySerializer implements KeySerializer<Integer> {