Remove medium from CS EE

What does this MR do?

• Makes Unknown default value for CS (container scanning) confidence field (it was Medium)
• Updates existing data with background migration

#13314 (closed)

Does this MR meet the acceptance criteria?

• CS should not display medium as confidence field

Conformity

• Changelog entry follow-up review issue created
• Code review guidelines(https://docs.gitlab.com/ee/development/merge_request_performance_guidelines.html)
• Style guides
• Database guides

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers