Relax version constraints for vitest

[ocavue]

Summary

@cloudflare/vitest-pool-workers supports the latest v2.1.x in this PR (cloudflare/workers-sdk#6716), so it might be a good idea to relax the version constraints in the documents too.

Screenshots (optional)

N/A

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[GregBrimble]

@penalosa , thoughts on this change?

[penalosa]

While we do currently work with the latest 2.x version of Vitest, there's no guarantee that'll hold in future (we rely on Vitest internals that aren't covered by semver). I think we should instead update the docs to mention 2.1.8 as the latest supported version.

[GregBrimble]

Thanks for your PR! However, given @penalosa's comment, I'm going to close this in favor of #18474. Appreciate the suggestion nevertheless :)