docs: deprecate benchmark.outputFile option

[hi-ogawa]

Description

• Closes [ benchmark ] unable to use the json reporter #5953

I thought we could remove it entirely, but there might be a need in the future to bring back the same code, so I only noted it's deprecated to avoid confusion.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

[netlify]

✅ Deploy Preview for vitest-dev ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	ba6e05c
🔍 Latest deploy log	https://app.netlify.com/sites/vitest-dev/deploys/66f8e17662aa360008a7e64b
😎 Deploy Preview	https://deploy-preview-6591--vitest-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.