Fix data in import_sources column of application_settings

What does this MR do and why?

Describe in detail what your merge request does and why.

Fix data in import_sources column of application_settings.

!118026 (merged) incorrectly saved import_sources as serialized string instead of as serialized YAML.

This MR:

• Fixes the original migration to properly store import_sources as serialized YAML.
• Introduces a new migration to convert the serialized string to YAML in any environments where the migration has already run.

Related to gitlab-com/gl-infra/production#9134 (closed).

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