### 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 routing;
        }
    }
    /**
     * Returns indexing routing for the given index.
     */
    
    
    public String resolveIndexRouting(@Nullable String routing, String aliasOrIndex) {
        if (aliasOrIndex == null) {
            return routing;
        }
        IndexAbstraction result = getIndicesLookup().get(aliasOrIndex);
        if (result == null || result.getType() != IndexAbstraction.Type.ALIAS) {
            return routing;
        }
        if (result.getIndices().size() > 1) {
            rejectSingleIndexOperation(aliasOrIndex, result);
        }
        return resolveRouting(routing, aliasOrIndex, AliasMetadata.getFirstAliasMetadata(this, result));
    }
    private static String resolveRouting(@Nullable String routing, String aliasOrIndex, AliasMetadata aliasMd) {
        if (aliasMd.indexRouting() != null) {
            if (aliasMd.indexRouting().indexOf(',') != -1) {
                throw new IllegalArgumentException(
                    "index/alias ["
                        + aliasOrIndex
                        + "] provided with routing value ["
                        + aliasMd.getIndexRouting()
                        + "] that resolved to several routing values, rejecting operation"