### 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
return field;
        }
        return path + "." + field;
    }
    /**
     * Returns all the concrete indices.
     */
    public String[] getConcreteAllIndices() {
        return allIndices;
    }
    /**
     * Returns all the concrete indices that are not hidden.
     */
    public String[] getConcreteVisibleIndices() {
        return visibleIndices;
    }
    /**
     * Returns all of the concrete indices that are open.
     */
    public String[] getConcreteAllOpenIndices() {
        return allOpenIndices;
    }
    /**
     * Returns all of the concrete indices that are open and not hidden.
     */
    public String[] getConcreteVisibleOpenIndices() {
        return visibleOpenIndices;
    }
    /**
     * Returns all of the concrete indices that are closed.