There's no 'group_relation' entity type.

Not to say that entity is used as boolean ($is_group_node).. No time to cleanup this though. Here's a patch.

I validated that a group_relationship entity exists via https://git.drupalcode.org/project/group/-/blob/3.3.x/src/Entity/GroupRe... and it was added in 2022 (@see #3303321: Drop 2.0.x upgrade path and update machine names → )

The code being updated was added via 📌 Support group v2/v3 Fixed
I would be happy to merge this is someone can confirm that we should have looking for the group_relationship entity and NOT the group_relation entity