Merge branch '21814-fix' into 'master'

Pull import sources from Gitlab::ImportSources

## What does this MR do?
`ApplicationSettings.create_from_defaults` defines an array of import sources, but we should be pulling them from `Gitlab::ImportSources.options` since that's what they're validated against.

## What are the relevant issue numbers?

Closes #21814

See merge request !6293