Update ssr.mdx

[zkuzmic]

What: Clarify documentation on SSR page

Why: The current wording makes it seem as if only the "Advanced Approach" does not support React 18 streaming APIs when in reality, emotion doesn't support those APIs at all (regardless of approach). See #2800

How: Added a note at the top of the doc page that indicates the above more clearly.

Checklist:

• Documentation
• Tests N/A
• Code complete N/A
• Changeset N/A

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Latest commit: 856f63e

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