revert: change to shiki behaviour

[ematipico]

Description (required)

Reverts some changes on the upgrade guide. Plus, I added a limitation to the CSP. I know users asked for this, and we decided we want to support it, I added a paragraph about it

For Astro version: 6.0.0. See astro PR #15640.

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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]

Noting that this appears to have already been released, so this is NOT merge-on-release and can merge whenever we are happy with it!

[sarah11918]

Config reference PR: withastro/astro#15658 for the updated JSDoc

[sarah11918]

This PR is fine! We've got the other PR for the config reference now!