fix(next): typo "v5" in v6 migration guide

[trueberryless]

Description

Under the Breaking changes section, the migration guide still says that these are breaking changes in "Astro v5.0", alto these are the breaking changes of Astro v6.0 in the v6 migration guide.

I noted that as well in this comment.

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[yanthomasdev]

Nice catch!