chore: update CSP docs for v5.9.2

[ematipico]

Description (required)

This PR updates the CSP docs where view transitions via ClientRouter aren't supported anymore

For Astro version: 5.9.2. See astro PR #13914.

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Tracked Files

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en/reference/experimental-flags/csp.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[sarah11918]

I guess this can merge at any time you want @ematipico , since all it does is recommend that people not use it with <ClientRouter> !