fix: add required vitest version upper boundary for vitest-plugin

[kol3x]

Summary

Sets an upper boundary on the Vitest version required by vitest-plugin in the corresponding note.

It currently states that "package requires Vitest 4.1 or later", but Vitest v5+ triggers a warning during test run (and failure in my case) on latest @cloudflare/vitest-plugin version:

You're running vitest@5.0.0, but this version of @cloudflare/vitest-plugin only officially supports vitest ^4.1.0.

Documentation checklist

• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

Review

✅ No issues found in commit f7e306d.

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Review coverage

🟡 1 ownership area needs approval.

Suggested reviewers: @for-the-kidz @irvinebroque @korinne

Needs approval	Eligible approvers	Suggested
workers	@GregBrimble, @irvinebroque, @korinne, @MattieTK, @mikenomitch, @vy-ton, @WalshyDev, @cloudflare/deploy-config, @cloudflare/dev-plat-leads, @cloudflare/workers-runtime-1, @cloudflare/wrangler, @cloudflare/product-owners	for-the-kidz, irvinebroque, korinne

CODEOWNERS mappings for displayed areas (1)

Pattern	Owners
/src/content/docs/workers/	@GregBrimble, @irvinebroque, @korinne, @MattieTK, @mikenomitch, @vy-ton, @WalshyDev, @cloudflare/deploy-config, @cloudflare/dev-plat-leads, @cloudflare/workers-runtime-1, @cloudflare/wrangler, @cloudflare/product-owners