Remove use of deprecated corepack

corepack was recently deprecated, as no one remaining on the nodejs core team wishes to continue maintaining it (the original author of it and only contributor to it has left the project).

it's also buggy, especially when it comes to `pnpm install`. I discovered we were using it for this repo when working to automate docs previews for `ember-data`.

```
$ node generate-local.js --project ember-data --version "5.4.0"
Running 'corepack' in ../data
/Users/cthoburn/.volta/tools/image/node/22.3.0/lib/node_modules/corepack/dist/lib/corepack.cjs:22460
throw new Error(`Cannot find matching keyid: ${JSON.stringify({ signatures, keys })}`);
```

googling tells me I can work around this particular bug by telling corepack to ignore integrity checks entirely if pnpm is in use, but since we also shouldn't be using corepack anymore I figured I'd open an issue.