Why?

We've introduced filter_attributes feature by #709.
However, it requires Rails 6.0 or greater, and we have the conditional code for it.

The severe security support for Rails 5.2 has been expired.
https://guides.rubyonrails.org/maintenance_policy.html#severe-security-issues

So we will drop Rails 5.2 or lower since mongomapper v0.16.