Enable github_importer_use_diff_note_with_suggestions by default

What does this MR do and why?

This feature was already tested in production, but since it might have performance impacts the feature flag won't be removed at this moment.

Related to:

• Feature request: #340624 (closed)
• Feature flag rollout: #344309 (closed)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.