docs(astro): update minimum Astro version to 4

[nicohrubec]

DESCRIBE YOUR PR

Set the Astro minimum to 4.0.0 for v11 and drop the now-moot 3.5.2 version qualifiers from the server-instrumentation notes.

⚠️ Do not merge until v11 is released as stable.

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Thanks in advance for your help!

PRE-MERGE CHECKLIST

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• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

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[linear-code]

SDK-1464