Consider dropping CommonJS support

## Dependencies dropping support

Several of our dependencies (and other major JS frameworks) have begun dropping support for CommonJS. Here are a few prominent examples relevant to us:

* `node-fetch` - Dropped polyfill for CommonJS after version 3.
* `uuid` - UUID dropped CommonJS after v11 (we were recently forced to remove this library due to vulnerabilities in older versions, we couldn't easily upgrade to newer versions).
* `p-queue`, `p-retry`, `p-map` - Current versions have dropped CJS

## Maintenance cost

In addition to being stuck on older library versions, maintaining both package formats creates complexity for our infrastructure (i.e. librarian, GAPIC generator). Removing the CJS support would enable us to delete a lot of code and reduce the maintenance cost.

## Ecosystem maturity

For our customers, migrating to support ESM has gotten a lot easier. ESM modules can be imported using the legacy `require` function. This significantly reduces the code changes required to consume an ESM library: https://joyeecheung.github.io/blog/2025/12/30/require-esm-in-node-js-from-experiment-to-stability/