Also warn on timestamp in datetime migration cop

What does this MR do?

Makes the datetime migration RuboCop cop also warn on timestamp columns.

Why was this MR needed?

Those names are aliases: https://github.com/rails/rails/blob/v4.2.10/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb#L362-L364

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together