Replace 'alias_method_chain' with 'Module#prepend'

What does this MR do?

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.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22302 (closed)