fix: update astro and starlight

[khendrikse]

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Summary

There were some breaking changes, thus this PR instead of a normal dependency update.

Replaces: #7158

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[github-actions]

📊 Benchmark results

Comparing with c25039c

• Dependency count: 1,105 (no change)
• Package size: 276 MB ⬆️ 0.00% increase vs. c25039c
• Number of ts-expect-error directives: 399 (no change)