Serialize renamed application setting asset_proxy_whitelist

What does this MR do?

This MR brings back the serialization code for the renamed application settings column (!50824 (merged)) because it is still needed to deserialize the column before actually renaming the column. Otherwise we see an exception like: gitlab-com/gl-infra/production#3494 (closed)

Screenshots (strongly suggested)

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