Add compatibility with Group 2

A version for Group 2.x was already prepared, but could not be released because it is untested. I have published a new release for Group 2.x, you are welcome to test it and if something does not fit, then make adjustments directly for the version.

https://www.drupal.org/project/group_storage/releases/2.0.0-beta1 →

I see. I am still a little confused because group_storage 1.x has composer support for group 2.x, while group_storage 3.x (and now 2.x) has quite a bit of different code. Even still 2.x and 3.x have notable differences that I don't have insight into. From my perspective the 2.x and 3.x versions of group_storage should be identical only with different ID values (group_content vs group_relationship) everywhere.

However, in the absence of my full understanding I'm going to just make two forks (one based on 3.x and one on 2.x), both targeting the 2.x branch with whatever changes I find are needed. You can choose the path forward you want.

3415707-add-compatibility-with (https://git.drupalcode.org/project/group_storage/-/merge_requests/6) is based off of 3.x and preserves its code changes while still using group 2.x IDs (group_content instead of group_relationship).

3415707-add-compatibility-2.x (https://git.drupalcode.org/project/group_storage/-/merge_requests/7) is based off of 2.x and just fixes the view configuration and doesn't change any code. All other code seemed to use the right IDs.