Problem/Motivation
The post update workspaces_post_update_move_association_data() makes an entity schema change. It deletes the workspace_association entity.
Generic entity type operations in hook_update_N implementations cannot depend on this update because it's a post update. If they are checking something on all entity types then the workspace_association entity type is completely invalid because neither the storage nor the entity type class exist.
As a whole workspaces_post_update_move_association_data() itself cannot be a hook_update_N because it is saving revisions.
Proposed resolution
Move the entity type deletion to an update hook so updates can depend on it.