Add BUG_IN_CODE comment to file locking quarantined spec

What does this MR do?

Adds BUG_IN_CODE comment to the quaranited file_locking_spec.rb according to the handbook

Follow-up to #43105 (comment 259796156)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content