elasticsearch can't serialize 6.1 volto data as it contains sets

[instification]

plone.volto creates a custom index for block_types which returns a set. This doesn't cause any issues for the portal catalog but the data can't be directly serialized to json.

see: https://github.com/plone/plone.volto/blob/63715cca4f6dc9129f1d9ab4d2f7d068620ec2e7/src/plone/volto/indexers.py#L39-L48

The data seems to just be a simple list, and possibly the best fix might be to question why it is a set.

I have a PR which I will link that serializes sets to lists when indexing to es and resolves the issue