fix: do not cache `/api/exit-preview`

[angeloashmore]

Types of changes

• [ ] Chore (a non-breaking change which is related to starter maintenance)
• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR ensures /api/exit-preview is not cached.

Before this PR, previews could not be exited unless caching was disabled in the browser (not the default).

Checklist:

• [ ] My change requires an update to the official documentation.
• [ ] All TSDoc comments are up-to-date and new ones have been added where necessary.
• [ ] All new and existing tests are passing.

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Name	Status	Preview	Updated
nextjs-starter-prismic-blog	✅ Ready (Inspect)	Visit Preview	Aug 16, 2022 at 1:22AM (UTC)

[angeloashmore]

@prismicio/next has since been updated to do what this PR implements.

See @prismicio/next's exitPreview() source here for details: https://github.com/prismicio/prismic-next/blob/cc825bd8ca36b4215880d61c7774ca9c10552f4b/src/exitPreview.ts#L43-L50