Use migration bot user in snippet backfilling migrations

What does this MR do?

There are several cases where the snippet cannot be created using the snippet author because maybe the user is blocked, for example. There are other scenarios where even admins can't update a snippet because the snippet feature is disabled or the project is archived.

In order to migrate all snippets and avoid having non-migrated and migrated ones, we need a way to ensure that we migrate all snippets.

In these scenarios, were policies avoid the snippet to be migrated, we're going to use the migration bot user as the creator of the repository (and the commit).

Refs #216120 (closed)

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