GithubImporter: Fallback to LegacyDiffNote when DiffNote fails

What does this MR do and why?

When DiffNote fails to create the diff note file with DiffNote::NoteDiffFileCreationError exception, fallsback to use the LegacyDiffNote.

This error, DiffNote::NoteDiffFileCreationError, is not exclusive on the import process, that's why we're falling back to the LegacyDiffNote model.

Related to: #346638 (closed)

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