Use alternatives to formData from undici for processing form data

[timtucker]

Undici v5 is EOL as of April 30, 2025.

In v6, the formData() method on responses was marked as deprecated.

It's still there in v7, but presumably will be dropped in a future release.

Azure Functions should use the approach suggested by Undici:
https://github.com/nodejs/undici/blob/v6.21.1/types/fetch.d.ts

/**
   * @deprecated This method is not recommended for parsing multipart/form-data bodies in server environments.
   * It is recommended to use a library such as [@fastify/busboy](https://www.npmjs.com/package/@fastify/busboy) as follows:
   *
   * @example
   * ```js
   * import { Busboy } from '@fastify/busboy'
   * import { Readable } from 'node:stream'
   *
   * const response = await fetch('...')
   * const busboy = new Busboy({ headers: { 'content-type': response.headers.get('content-type') } })
   *
   * // handle events emitted from `busboy`
   *
   * Readable.fromWeb(response.body).pipe(busboy)
   * ```
   */
  readonly formData: () => Promise<FormData>

This issue goes into the reasons why they suggest using the approach above:
nodejs/undici#2890