Exclude migration models from DB dictionary

What does this MR do and why?

In order to decouple migrations from the application code we define AR models inside migration when needed (sub-classes of MigrationRecord). These are usually not included in the DB dictionary, as the files are not loaded, unless it happens the dictionary is generated after db:migrate that executed migration defining such model. In this case we end up with the model added to the classes list for the table.

This MR adds additional filter in order to reject such model classes.

Screenshots or screen recordings

How to set up and validate locally

1. Create migration # db/migrate/20230303000404_test_with_model.rb like:

   class TestWithModel < Gitlab::Database::Migration[2.1]
     class Project < MigrationRecord
       self.table_name = 'projects'
     end
   
     def change
     end
   end

2. When on master execute db:migrate. There should be a diff like:

   diff --git a/db/docs/projects.yml b/db/docs/projects.yml
   index 0b7dbbe2127c..82cb3c64a4f4 100644
   --- a/db/docs/projects.yml
   +++ b/db/docs/projects.yml
   @@ -2,6 +2,7 @@
    table_name: projects
    classes:
    - Project
   +- TestWithModel::Project
    feature_categories:
    - projects
    description: Stores project records

3. When on this branch execute db:migrate. There should no change to db/docs/projects.yml.

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