### 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
mapBuilder.put(index, Collections.unmodifiableList(filteredValues));
            }
        }
        return mapBuilder.build();
    }
    /**
     * Finds all mappings for concrete indices. Only fields that match the provided field
     * filter will be returned (default is a predicate that always returns true, which can be
     * overridden via plugins)
     *
     * @see MapperPlugin#getFieldFilter()
     *
     * @param onNextIndex a hook that gets notified for each index that's processed
     */
    public Map<String, MappingMetadata> findMappings(
        String[] concreteIndices,
        Function<String, Predicate<String>> fieldFilter,
        Runnable onNextIndex
    ) {
        assert Transports.assertNotTransportThread("decompressing mappings is too expensive for a transport thread");
        assert concreteIndices != null;
        if (concreteIndices.length == 0) {
            return ImmutableOpenMap.of();
        }
        ImmutableOpenMap.Builder<String, MappingMetadata> indexMapBuilder = ImmutableOpenMap.builder();
        Set<String> indicesKeys = indices.keySet();
        Stream.of(concreteIndices).filter(indicesKeys::contains).forEach(index -> {
            onNextIndex.run();
            IndexMetadata indexMetadata = indices.get(index);
            Predicate<String> fieldPredicate = fieldFilter.apply(index);