RSpec: Infer migration filename also for spec files with schema version

What does this MR do and why?

Prior this MR you had to pass a filename to require_migration! in cases where your spec included "schema version" in its filename.

With this MR the helper method is able to infer migration filename even in these scenarios.

Caveats

Heads-up! Due to typos we cannot apply the change to the following files which caused a 🔴 pipeline:

• spec/migrations/cleanup_move_container_registry_enabled_to_project_features_spec.rb - https://gitlab.com/gitlab-org/gitlab/-/jobs/1679913313
• spec/migrations/fix_projects_without_prometheus_services_spec.rb - https://gitlab.com/gitlab-org/gitlab/-/jobs/1679913068
• spec/migrations/remove_duplicate_labels_from_groups_spec.rb - https://gitlab.com/gitlab-org/gitlab/-/jobs/1679913052

MR acceptance checklist

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