Breaking changes in mongoid 7 support commit

Hi,

Unfortunately, there is breaking change in [Add macro to support mongoid 7](https://github.com/activeadmin/activeadmin-mongoid/commit/3e6b8d84a56bea408165b6e23f6f6c303f6cdbc2) commit.
Starting from this commit, mongoid 6.x starts to crash with `uninitialized constant Mongoid::Association` error, since it doesn't have this module.

If you would like, we can discuss way of fixing it, and I'll prepare PR