Remove legacy data related to CI minutes after monthly CI usage tracked

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Clean up task - after Monthly CI Usage tracking has been fully 🚢

• Remove columns namespace_statistics.shared_runners_seconds and namespace_statistics.shared_runners_seconds_last_reset
• Remove columns project_statistics.shared_runners_seconds and project_statistics.shared_runners_seconds_last_reset
• Remove columns namespaces.last_ci_minutes_notification_at and namespaces.last_ci_minutes_usage_notification_level

There will be three releases to drop columns, see https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-columns:

Step	MR	Status
Ignoring the column	!117975	workflowin review
Dropping the column
Removing the ignore rule