[Workers] Update vitest-pool-workers docs to newer vitest version

[cjlawson02]

Summary

Update docs to install 3.1.x of vitest, and update compatibility note.

Relevant compatibility change: cloudflare/workers-sdk#8804

Documentation checklist

• The documentation style guide has been adhered to.

[hyperlint-ai]

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PR Change Summary

Updated documentation for vitest-pool-workers to reflect compatibility with the newer vitest version.

• Updated installation instructions for vitest to version 3.1.x
• Revised compatibility note to include support for vitest 3.1.x

Modified Files

• src/content/docs/workers/testing/vitest-integration/write-your-first-test.mdx

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[cjlawson02]

Duplicate of #21700