Rails/MigrationClassName triggers on classes that are not migrations

[tomhughes]

Expected behavior

The Rails/MigrationClassName cop should only trigger on the migration class that inherits from ActiveRecord::Migration.

Actual behavior

The Rails/MigrationClassName cop will trigger on top level classes which are supporting classes that are not the migration.

Steps to reproduce the problem

Consider this example:

class NotAMigration
end

class ThisMigration < ActiveRecord::Migration[4.2]
end

Only the second class should be affected by the cop but in fact the first one is and the second one is left alone.

RuboCop version

1.26.0 (using Parser 3.1.1.0, rubocop-ast 1.16.0, running on ruby 3.0.2 x86_64-linux)
  - rubocop-minitest 0.18.0
  - rubocop-performance 1.13.3
  - rubocop-rails 2.14.1
  - rubocop-rake 0.6.0