Update DynamicModelHelpers to require a connection

What does this MR do and why?

Related issue: #345624 (closed)

Update DynamicModelHelpers methods to set the model's database connection to the given connection rather than using the default connection from ActiveRecord::Base. This was the suggested solution in the discussion at !77717 (comment 812876439)

Since these methods are commonly used in migrations, I added wrappers which use the migration connection. I wanted to keep the connection: argument in DynamicModelHelpers required, so it's explicit which connection non-migration code is using.

It was also mentioned there that the DynamicModelHelpers might cause a memory leak when it creates anonymous ActiveRecord models. It seems like that issue was fixed in https://github.com/rails/rails/pull/31442. Locally it seems to work alright:

[1] pry(main)> ActiveRecord::Base.descendants.size
=> 140
[2] pry(main)> 100.times { Class.new(ActiveRecord::Base) }
=> 100
[3] pry(main)> ActiveRecord::Base.descendants.size
=> 240
[4] pry(main)> GC.start
=> nil
[5] pry(main)> ActiveRecord::Base.descendants.size
=> 140

How to set up and validate locally

You can validate this locally if you have your environment setup with multiple databases.

1. Create a test object with the module:

   model_builder = Class.new do
     include Gitlab::Database::DynamicModelHelpers
   end.new

2. Query with a dynamic model pointed at the main database:

   model_builder.define_batchable_model('projects', connection: Project.connection).count # => 8

3. Query with a dynamic model pointed at the ci database:

   model_builder.define_batchable_model('projects', connection: Ci::ApplicationRecord.connection).count # => 0

MR acceptance checklist

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