### 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
private static final Logger logger = LogManager.getLogger(Metadata.class);
    public static final Runnable ON_NEXT_INDEX_FIND_MAPPINGS_NOOP = () -> {};
    public static final String ALL = "_all";
    public static final String UNKNOWN_CLUSTER_UUID = "_na_";
    public enum XContentContext {
        
        API,