Switch to Ci::ApplicationRecord for this BG migration

What does this MR do and why?

Switch to using Ci::ApplicationRecord.connection for this BG migration (Gitlab::BackgroundMigration::NullifyOrphanRunnerIdOnCiBuilds)

This ensures that this BG migration will work for decomposed databases as well

Related issue: #359952 (closed)

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How to set up and validate locally

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MR acceptance checklist

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