docs: add gaxios and node 18 migration guide

Author: thiyaguk09

handwritten/storage/MIGRATION.md

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+# Storage v7 to v8 Migration Guide - Node.js 18 & Gaxios Update
+
+This guide helps you migrate your application from `@google-cloud/storage` v7 to v8, focusing on the transition to Node.js 18+ as the minimum supported environment and changes introduced by updating the underlying HTTP client, `gaxios`, to version 7.
+
+## Minimum Requirements: Node.js 18
+
+`@google-cloud/storage` v8 now officially requires Node.js 18 or higher. This update aligns the library with modern JavaScript environments and allows us to leverage standard native web APIs like the global `Headers` constructor.
+
+## Key Breaking Changes for Storage Users
+
+### 1. Response Headers are now `Headers` objects
+
+When you receive a full API response from Storage methods (e.g., via callbacks or promise resolutions that include the response object), the `headers` property of the response is now a standard native `Headers` object (aligned with the Fetch API standard in Node.js 18) rather than a plain JavaScript object.
+
+**Before (Storage v7):**
+
+```js
+const [retrievedFile, apiResponse] = await file.get();
+const contentType = apiResponse.headers['content-type'];
+```
+
+**After (Storage v8):**
+
+```js
+const [retrievedFile, apiResponse] = await file.get();
+// Accessing headers requires the .get() method
+const contentType = apiResponse.headers.get('content-type');
+```
+
+### 2. Passing Headers in Options
+
+If you pass custom headers in options to Storage methods (which extend `GaxiosOptions`), you can still pass plain objects, as the Storage library will convert them to standard `Headers` internally for the request. However, if you read them back from the prepared options or response, they will be `Headers` objects.
+
+### 3. URL Resolution (`baseURL`)
+
+If you are using custom `baseURL` options or passing relative URLs to methods that accept them, be aware that resolution now strictly follows the standard native `URL` constructor spec (`new URL(url, baseURL)`).
+
+## Upgrade Instructions
+
+Update your `@google-cloud/storage` dependency to version 8:
+
+```sh
+npm install @google-cloud/storage@latest
+```
+
+## Troubleshooting
+
+- If you encounter `undefined` when accessing headers on the response object, ensure you are using `apiResponse.headers.get('header-name')`.