### 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 true if this custom could be restored from snapshot
         */
        default boolean isRestorable() {
            return context().contains(XContentContext.SNAPSHOT);
        }
    }
    public static final Setting<Boolean> SETTING_READ_ONLY_SETTING = Setting.boolSetting(
        "cluster.blocks.read_only",
        false,
        Property.Dynamic,
        Property.NodeScope
    );
    public static final ClusterBlock CLUSTER_READ_ONLY_BLOCK = new ClusterBlock(
        6,
        "cluster read-only (api)",
        false,
        false,
        false,
        RestStatus.FORBIDDEN,
        EnumSet.of(ClusterBlockLevel.WRITE, ClusterBlockLevel.METADATA_WRITE)
    );
    public static final Setting<Boolean> SETTING_READ_ONLY_ALLOW_DELETE_SETTING = Setting.boolSetting(
        "cluster.blocks.read_only_allow_delete",
        false,
        Property.Dynamic,
        Property.NodeScope
    );
    public static final ClusterBlock CLUSTER_READ_ONLY_ALLOW_DELETE_BLOCK = new ClusterBlock(
        13,
        "cluster read-only / allow delete (api)",