Use migration class for create table template

What does this MR do and why?

Adjust the template for generating "create table migrations" to use Gitlab::Database::Migration[1.0] as a superclass instead of ActiveRecord::Migration[6.1].

See !68986 (comment 679091141)

What is not covered here is model scaffolding, e.g. rails generate model Product name:string description:text would still use the original AR template.

How to set up and validate locally

rails g migration CreateFooTable foo:string

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.