[13.0.0] Remove references to public_group, open_group, closed_group, and secret_group for social_post module

Problem/Motivation

We're removing the old group types from Open Social. This module has some references to the old group types which should be removed.

Steps to reproduce

Proposed resolution

Evaluate the references to the old group types. We might be able to remove the functionality that exists around it altogether and remove the dependency on any group type. If that's not possible just remove the references to the old group type. In any case the functionality itself should keep working for flexible groups.

Remaining tasks

User interface changes

API changes

Data model changes