Change gitlab_schema default value in have_scheduled_batched_migration matcher

What does this MR do and why?

This MR does the following:

• Changes the default value of the :gitlab_schema attribute from :gitlab_main to nil, and defaults to the schema specified in the migration itself through restrict_gitlab_migration.
• Adds missing :migration attributes to migrations as per the documentation (https://docs.gitlab.com/ee/development/testing_guide/testing_migrations_guide.html#how-does-it-work).
• Updates the template to add the missing attribute.

Closes #494933 (closed)

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