chore: document major shiki styles

[ematipico]

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This PR documents the breaking change of withastro/astro#15451

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For Astro version: 6.0. See astro PR #15451.

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en/guides/upgrade-to/v6.mdx	Localization changed, will be marked as complete.

Warnings reference

Icon	Description
🔄️	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]

Thanks @ematipico ! Just formatted this one up similar to the other one, but also, we should move this up into the ## Changed Defaults category of changes (like I did with the responsive images/csp one). I think this one fits directly after that one!