Remove the "soon to be deprecated" `alias_method_chain` in favor of `Module#prepend`.

## Are there points in the code the reviewer needs to double check?

Double check whether the behavior of `attr_encrypted_no_db_connection` is still the desired one.

## Why was this MR needed?

The `alias_method_chain` becomes deprecated in Rails 5 in favor of the `Module#prepend` introduced in Ruby 2.0.

This MR prevents future deprecated warnings in light of a possible Rails version bump.

Closes #22302

See merge request !6570