[Workers] fix: update supported vitest version (#18474)

* fix: update supported vitest version

* chore: pin vitest to 2.1.5

This works around vitest-dev/vitest#6992 being an issue with Vite 6

* chore: update vitest to 2.1.8

* chore: update vitest to 2.1.8

* Update src/content/docs/workers/testing/vitest-integration/get-started/write-your-first-test.mdx

---------

Co-authored-by: Greg Brimble <developer@gregbrimble.com>

src/content/docs/workers/testing/vitest-integration/get-started/migrate-from-miniflare-2.mdx

@@ -30,7 +30,7 @@ First, you will need to uninstall the old environment and install the new pool.
 
 ```sh
 npm uninstall vitest-environment-miniflare
-npm install --save-dev --save-exact vitest@2.0.5
+npm install --save-dev --save-exact vitest@2.1.8
 npm install --save-dev @cloudflare/vitest-pool-workers
 ```
 

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

@@ -24,15 +24,15 @@ This guide will instruct you through installing and setting up the `@cloudflare/
 Open a terminal window and make sure you are in your project's root directory. Once you have confirmed that, run:
 
 ```sh
-npm install vitest@2.0.5 --save-dev --save-exact
+npm install vitest@2.1.8 --save-dev --save-exact
 npm install @cloudflare/vitest-pool-workers --save-dev
 ```
 
 The above commands will add the packages to your `package.json` file and install them as dev dependencies.
 
 :::note
 
-Currently, the `@cloudflare/vitest-pool-workers` package _only_ works with Vitest 2.0.5.
+Currently, the `@cloudflare/vitest-pool-workers` package _only_ works with Vitest 2.0.x - 2.1.8.
 
 :::